Required fixes:
- CallbackQuery is now comparable.
- Message.effective_attachment, Message.photo,
Message.new_chat_members, Message.new_chat_photo &
Game.text_entitties semantic fixes - when they are not defined,
return an empty list.
- Docstring fix to Update class.
* Small fixes to documentation and add myself to AUTHORS.
* Rework CONTRIBUTIONS.rst
Use code-blocks instead of literals, change headings for portability and use a relative link to AUTHORS instead of linking to a specific copy.
