Added Discourse to the list and small fixes

Other small fixes include: 

* Both readme and index supported sites lists are in the same order and have the same sites
* And Bang was in readme with a broken link, but not in index. Fixed link and added to index.
* Consistent use of the oxford comma
* Supported sites list and "However some of the emoji codes are not super easy to remember..." have their own respective paragraph tags 
* Added myself to contributors list 

:cake: