Docs: Add documentation about chatmessages

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+# ChatMessages
+
+ChatMessages are the messages sent in 1-on-1 chats. They are similar to
+`Note`s, but the addresing is done by having a single AP actor in the `to`
+field. Addressing multiple actors is not allowed. These messages are always
+private, there is no public version of them. They are created with a `Create`
+activity.
+
+Example:
+
+```json
+{
+  "actor": "http://2hu.gensokyo/users/raymoo",
+  "id": "http://2hu.gensokyo/objects/1",
+  "object": {
+    "attributedTo": "http://2hu.gensokyo/users/raymoo",
+    "content": "You expected a cute girl? Too bad.",
+    "id": "http://2hu.gensokyo/objects/2",
+    "published": "2020-02-12T14:08:20Z",
+    "to": [
+      "http://2hu.gensokyo/users/marisa"
+    ],
+    "type": "ChatMessage"
+  },
+  "published": "2018-02-12T14:08:20Z",
+  "to": [
+    "http://2hu.gensokyo/users/marisa"
+  ],
+  "type": "Create"
+}
+```
+
+This setup does not prevent multi-user chats, but these will have to go through
+a `Group`, which will be the recipient of the messages and then `Announce` them
+to the users in the `Group`.