Add ROADMAP.md with approximate current state

ROADMAP.md

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+# Features & roadmap
+* Matrix → Discord
+  * [x] Message content
+    * [x] Plain text
+    * [ ] Formatted messages
+    * [x] Media/files
+    * [x] Replies
+    * [ ] Threads
+  * [x] Message redactions
+  * [x] Reactions
+    * [x] Unicode emojis
+    * [ ] Custom emojis
+  * [ ] Presence
+  * [ ] Typing notifications
+  * [ ] Own read status
+  * [ ] Power level
+  * [ ] Membership actions
+    * [ ] Invite
+    * [ ] Leave
+    * [ ] Kick
+  * [ ] Room metadata changes
+    * [ ] Name
+    * [ ] Avatar
+    * [ ] Topic
+  * [ ] Initial room metadata
+* Discord → Matrix
+  * [ ] Message content
+    * [x] Plain text
+    * [ ] Formatted messages
+    * [x] Media/files
+    * [x] Replies
+    * [ ] Threads
+  * [x] Message deletions
+  * [ ] Reactions
+    * [x] Unicode emojis
+    * [ ] Custom emojis
+  * [x] Avatars
+  * [ ] Presence
+  * [ ] Typing notifications
+  * [ ] Membership actions
+    * [ ] Invite
+    * [ ] Join
+    * [ ] Leave
+    * [ ] Kick
+  * [ ] Channel/group DM metadata changes
+    * [ ] Title
+    * [ ] Avatar
+    * [ ] Description
+  * [ ] Initial channel/group DM metadata
+  * [ ] User metadata changes
+    * [ ] Display name
+    * [ ] Avatar
+  * [ ] Initial user metadata
+    * [ ] Display name
+    * [ ] Avatar
+* Misc
+  * [ ] Login methods
+    * [x] QR scan from mobile
+    * [ ] Manually providing access token
+  * [ ] Automatic portal creation
+    * [ ] After login
+    * [x] When receiving DM
+  * [ ] Private chat creation by inviting Matrix puppet of Discord user to new room
+  * [x] Option to use own Matrix account for messages sent from other Discord clients