Docs: Add chat motivation and api description.

docs/API/chats.md

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+# Chats
+
+Chats are a way to represent an IM-style conversation between two actors. They are not the same as direct messages and they are not `Status`es, even though they have a lot in common.
+
+## Why Chats?
+
+There are no 'visibility levels' in ActivityPub, their definition is purely a Mastodon convention. Direct Messaging between users on the fediverse has mostly been modeled by using ActivityPub addressing following Mastodon conventions on normal `Note` objects. In this case, a 'direct message' would be a message that has no followers addressed and also does not address the special public actor, but just the recipients in the `to` field. It would still be a `Note` and is presented with other `Note`s as a `Status` in the API.
+
+This is an awkward setup for a few reasons:
+
+- As DMs generally still follow the usual `Status` conventions, it is easy to accidentally pull somebody into a DM thread by mentioning them. (e.g. "I hate @badguy so much")
+- It is possible to go from a publicly addressed `Status` to a DM reply, back to public, then to a 'followers only' reply, and so on. This can be become very confusing, as it is unclear which user can see which part of the conversation.
+- The standard `Status` format of implicit addressing also leads to rather ugly results if you try to display the messages as a chat, because all the recipients are always mentioned by name in the message.
+- As direct messages are posted with the same api call (and usually same frontend component) as public messages, accidentally making a public message private or vice versa can happen easily. Client bugs can also lead to this, accidentally making private messages public.
+
+As a measure to improve this situation, the `Conversation` concept and related Pleroma extensions were introduced. While it made it possible to work around a few of the issues, many of the problems remained and it didn't see much adoption because it was too complicated to use correctly. 
+
+## Chats explained
+For this reasons, Chats are a new and different entity, both in the API as well as in ActivityPub. A quick overview:
+
+- Chats are meant to represent an instant message conversation between two actors. For now these are only 1-on-1 conversations, but the other actor can be a group in the future.
+- Chat messages have the ActivityPub type `ChatMessage`. They are not `Note`s. Servers that don't understand them will just drop them.
+- The only addressing allowed in `ChatMessage`s is one single ActivityPub actor in the `to` field.
+- There's always only one Chat between two actors. If you start chatting with someone and later start a 'new' Chat, the old Chat will be continued.
+- `ChatMessage`s are posted with a different api, making it very hard to accidentally send a message to the wrong person.
+- `ChatMessage`s don't show up in the existing timelines.
+- Chats can never go from private to public. They are always private between the two actors.
+
+## Caveats
+
+- Chats are NOT E2E encrypted (yet). Security is still the same as email.
+
+## API
+
+In general, the way to send a `ChatMessage` is to first create a `Chat`, then post a message to that `Chat`. The actors in the API are generally given by their ActivityPub id to make it easier to support later `Group` scenarios. 
+
+This is the overview of using the API. The API is also documented via OpenAPI, so you can view it and play with it by pointing SwaggerUI or a similar OpenAPI tool to `https://yourinstance.tld/api/openapi`.
+
+### Creating or getting a chat.
+
+To create or get an existing Chat for a certain recipient (identified by AP ID)
+you can call:
+
+`POST /api/v1/pleroma/chats/by-ap-id/{ap_id}`
+
+The ap_id of the recipients needs to be www-form encoded, so 
+
+```
+https://originalpatchou.li/user/lambda
+```
+
+would become
+
+```
+https%3A%2F%2Foriginalpatchou.li%2Fuser%2Flambda
+```
+
+The full call would then be
+
+```
+POST /api/v1/pleroma/chats/by-ap-id/https%3A%2F%2Foriginalpatchou.li%2Fuser%2Flambda
+```
+
+There will only ever be ONE Chat for you and a given recipient, so this call
+will return the same Chat if you already have one with that user.
+
+Returned data:
+
+```json
+{
+  "recipient" : "https://dontbulling.me/users/lain",
+  "recipient_account": {
+    "id": "someflakeid",
+    "username": "somenick",
+    ...
+  },
+  "id" : "1",
+  "unread" : 2
+}
+```
+
+### Getting a list of Chats
+
+`GET /api/v1/pleroma/chats`
+
+This will return a list of chats that you have been involved in, sorted by their
+last update (so new chats will be at the top).
+
+Returned data:
+
+```json
+[
+   {
+      "recipient" : "https://dontbulling.me/users/lain",
+      "recipient_account": {
+        "id": "someflakeid",
+        "username": "somenick",
+        ...
+      },
+      "id" : "1",
+      "unread" : 2
+   }
+]
+```
+
+The recipient of messages that are sent to this chat is given by their AP ID.
+The usual pagination options are implemented.
+
+### Getting the messages for a Chat
+
+For a given Chat id, you can get the associated messages with
+
+`GET /api/v1/pleroma/chats/{id}/messages`
+
+This will return all messages, sorted by most recent to least recent. The usual
+pagination options are implemented
+
+Returned data:
+
+```json
+[
+  {
+    "actor": "https://dontbulling.me/users/lain",
+    "chat_id": "1",
+    "content": "Check this out :firefox:",
+    "created_at": "2020-04-21T15:11:46.000Z",
+    "emojis": [
+      {
+        "shortcode": "firefox",
+        "static_url": "https://dontbulling.me/emoji/Firefox.gif",
+        "url": "https://dontbulling.me/emoji/Firefox.gif",
+        "visible_in_picker": false
+      }
+    ],
+    "id": "13"
+  },
+  {
+    "actor": "https://dontbulling.me/users/lain",
+    "chat_id": "1",
+    "content": "Whats' up?",
+    "created_at": "2020-04-21T15:06:45.000Z",
+    "emojis": [],
+    "id": "12"
+  }
+]
+```
+
+### Posting a chat message
+
+Posting a chat message for given Chat id works like this:
+
+`POST /api/v1/pleroma/chats/{id}/messages`
+
+Parameters:
+- content: The text content of the message
+
+Currently, no formatting beyond basic escaping and emoji is implemented, as well as no
+attachments. This will most probably change.
+
+Returned data:
+
+```json
+{
+  "actor": "https://dontbulling.me/users/lain",
+  "chat_id": "1",
+  "content": "Check this out :firefox:",
+  "created_at": "2020-04-21T15:11:46.000Z",
+  "emojis": [
+    {
+      "shortcode": "firefox",
+      "static_url": "https://dontbulling.me/emoji/Firefox.gif",
+      "url": "https://dontbulling.me/emoji/Firefox.gif",
+      "visible_in_picker": false
+    }
+  ],
+  "id": "13"
+}
+```
+
+### Notifications
+
+There's a new `pleroma:chat_mention` notification, which has this form:
+
+```json
+{
+  "id": "someid",
+  "type": "pleroma:chat_mention",
+  "account": { ... } // User account of the sender,
+  "chat_message": {
+    "chat_id": "1",
+    "id": "10",
+    "content": "Hello",
+    "actor": "https://dontbulling.me/users/lain"
+  },
+  "created_at": "somedate"
+}
+```