Contributor updates.

* Move Mark Hills to "Special Thanks".
* Remove duplicate "Alex Barker".
* Add some missing contributors from the 1.12.0 milestone page:
 - Kevin Wern
 - Varun Jewalikar
 - Keith Salisbury
 - Irina Grosu
 - Callum Styan
 - Rahul Behl
* Move Dennis Wallace to this-release contributors.