Add PROCESS.md

GitLab CE/EE are moving away from feature freeze and the process is
changing
https://gitlab.com/gitlab-org/release/docs/blob/21cbd409dd5f157fe252f254f3e897f01908abe2/general/deploy/auto-deploy-transition.md
which leads into GitLab Runner following a different process which is
not longer documented anywhere. The initial commit would be just
copy/pasting the PROCESS.md from
https://gitlab.com/gitlab-org/gitlab-ce/blob/11-11-stable/PROCESS.md
which specifies all the detail about feature freeze and release process.
5 jobs for docs/add-process-file in 5 minutes and 51 seconds
Status Job ID Name Coverage
  Prebuild
passed #228830311
check feature flags docs

00:01:26

manual #228830310
allowed to fail manual
review-docs-deploy
 
  Test
passed #228830313
docs check links

00:01:06

passed #228830312
allowed to fail
license_management

00:04:25

 
  Cleanup
manual #228830314
allowed to fail manual
review-docs-cleanup