Document how to create preparation MRs

What

• Creates https://gitlab.com/gitlab-org/release-tools/blob/jej-picking-into-merge-requests/doc/picking-into-merge-requests.md to outline process of creating preparation MRs and merging CE->EE
• Adds steps to patch.md.erb template
• Creates bundle exec rake "patch_branch[X.Y.Z]" rake task for generating MR description https://gitlab.com/gitlab-org/release-tools/merge_requests/173

Why

Over the last two release cycles we've moved to a process of creating merge requests to which changes are cherry-picked. There was no documentation for this so I wrote myself some notes and have cleaned them up a bit in this MR.

I've also added a rake task generate MR description, as copying previous descriptions and updating branch names / repos was tedious. https://gitlab.com/gitlab-org/release-tools/merge_requests/173

Related

Closes https://gitlab.com/gitlab-org/release-tools/issues/85