[TASK] Document how to contribute to Emogrifier.
[TASK] Document how to contribute to Emogrifier.

This change also add a table of contents to the Readme.

jjriv/emogrifier,sanderkruger/emogrifier,OzzyCzech/emogrifier,ricardofiorani/emogrifier,Seldaek/emogrifier,Synchro/emogrifier,iFixit/emogrifier,ernsto/emogrifier,MyIntervals/emogrifier