Add things to TODO.md.

TODO.md

@@ -1,6 +1,9 @@
 Things to do:
 -------------
 
+ - Define error type for ProtoDecoder errors.
+ - Remove BytesMut dependency from ProtoEncoder, use Vec<u8> instead.
+ - Remove dependency on bytes crate entirely.
  - Handle RoomLeaveRequest/Response.
  - Print out surplus bytes in hex to make analyzing them easier
  - Handle client connections