Mastodonplus.py/DEVELOPMENT.md

Here's some general stuff to keep in mind, and some work that needs to be done

* Mastodon.py tries to work for python2 as well as python3, so avoid things like annotations,
  use requests over urllib, et cetera.

* Current TODOs:
    * Stream since_id and better resume
    * Testing - test 2.3 stuff and verify it works, test pinning
    * 2.4 support:
        * Dict updates (bot flag, profile meta fields, ...?)
        * Push API