Merge branch 'process' into 'master'

Add PROCESS.md

See merge request gitlab-org/gitlab-workhorse!272

PROCESS.md

+# GitLab-Workhorse development process
+
+## Maintainers
+
+GitLab-Workhorse has the following maintainers:
+
+- Nick Thomas `@nick.thomas`
+- Jacob Vosmaer `@jacobvosmaer-gitlab`
+
+This list is defined at https://about.gitlab.com/team/.
+
+## Merging and reviewing contributions
+
+Contributions must be reviewed by at least one Workhorse maintainer.
+The final merge must be performed by a maintainer.
+
+## Releases
+
+New versions of Workhorse can be released by one of the Workhorse
+maintainers. The release process is:
+
+- create a merge request to update CHANGELOG and VERSION on the
+  respective release branch (usually `master`)
+- merge the merge request
+- run `make release` on the release branch