What does this MR do?
Add PROCESS.md to define the feature freeze process.
Why was this MR needed?
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.