add info about UX label to feature proposal to CONTRIBUTING.md

See the general Documentation guidelines http://docs.gitlab.com/ce/development/doc_styleguide.html

What does this MR do?

Adds information about adding the UX label to feature proposals. Many community contributions make it to the MR stage without any input from UX. Getting UX eyes on it early can make sure it does not get kicked back at the last minute due to UX inconsistencies/concerns.