Clarify documentation on how to add custom Omniauth provider to GitLab

Conflicts:
	app/contexts/filter_context.rb
	app/contexts/search_context.rb
	app/models/merge_request.rb
	app/models/note.rb
	app/views/shared/_merge_requests.html.haml
	spec/controllers/commit_controller_spec.rb
	spec/services/notification_service_spec.rb

Add guidelines for the Contributing Process in PROCESS.md