Add documentation for info API

docs/api.md

@@ -0,0 +1,52 @@
+# API
+
+There is a web API for getting information about things that are bridged with Out Of Your Element.
+
+The base URL is the URL of the particular OOYE instance, for example, https://bridge.cadence.moe.
+
+No authentication is required.
+
+I'm happy to add more endpoints, just ask for them.
+
+## Endpoint: GET /api/message
+
+|Query parameter|Type|Description|
+|---------------|----|-----------|
+|`message_id`|regexp `/^[0-9]+$/`|Discord message ID to look up information for|
+
+Response:
+
+```typescript
+{
+  source: "matrix" | "discord"                // Which platform the message originated on
+  matrix_author?: {                             // Only for Matrix messages; should be up-to-date rather than historical data
+    displayname: string,                        // Matrix user's current display name
+    avatar_url: string | null,                  // Absolute HTTP(S) URL to download the Matrix user's current avatar
+    mxid: string                                // Matrix user ID, can never change
+  },
+  events: [                                   // Data about each individual event
+    {
+      metadata: {                               // Data from OOYE's database about how bridging was performed
+        sender: string,                           // Same as matrix user ID
+        event_id: string,                         // Unique ID of the event on Matrix, can never change
+        event_type: "m.room.message" | string,    // Event type
+        event_subtype: "m.text" | string | null,  // For m.room.message events, this is the msgtype property
+        part: 0 | 1,                              // For multi-event messages, 0 if this is the first part
+        reaction_part: 0 | 1,                     // For multi-event messages, 0 if this is the last part
+        room_id: string,                          // Room ID that the event was sent in, linked to the Discord channel
+        source: number
+      },
+      raw: {                                    // Raw historical event data from the Matrix API. Contains at least these properties:
+        content: any,                             // The only non-metadata property, entirely client-generated
+        type: string,
+        room_id: string,
+        sender: string,
+        origin_server_ts: number,
+        unsigned?: any,
+        event_id: string,
+        user_id: string
+      }
+    }
+  ]
+}
+```