Cookbook on how to make a new fixer

Addresses FriendsOfPHP/PHP-CS-Fixer#682
I would add here (to assumptions section) sth like `you know the basis of... or want to become familiar` - not to scare people.

When I was starting working on this project I didn't know how to make fun with git, and know why TDD is cool but never used it for real, I was learning PHPUnit on this project.

It is cool if sb know all of it - but if no, the community will help ;)