Cookbook on how to make a new fixer

Addresses FriendsOfPHP/PHP-CS-Fixer#682
You know that issue was closed down. I just want to get all the best we can get.
But yeah, it may be short not to discourage people and maybe some day we add a cookbook level 2 or sth...

One way or another when using Tokens class one get different tokens than using token_get_all. It is worth at least one statement to write it down and point to Transformers - for the people not going crazy on different collection than they expected.