Mastodon.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.

* Unimplemented methods:
    * None, at time of writing!
    
* Documentation that needs to be updated:
    * Some things are probably wrong, but none known.
    
* Other things missing:
    * Actual error handling that is good: Proper dealing with OAuth errors, more specific exceptions