Add functional review process description (#334)

Author: acka47

CONTRIBUTING.md

@@ -184,6 +184,15 @@ After leaving those comments the reviewer removes their own assignment and lets
 After that the reviewer might be reassigned for a new review.
 If everything works as expected, the reviewer posts a +1 comment on the issue, approves the PR, removes their assignment and adds the suggested code reviewer as assignee and reviewer of the linked pull request.
 
+The concrete process for functional review at https://test.skohub.io looks like this:
+
+1. We merge the feature branches into the `dev` branch.
+2. This triggers a rebuild of test.skohub.io based on the `dev` branch.
+3. If needed, the reviewer adds or adjusts some vocabs in the [testing-skohub-vocabs repo](https://github.com/skohub-io/testing-skohub-vocabs) or otherwise only triggers a re-build with an empty commit (`$ git commit --allow-empty -m "Trigger webhook"').
+4. The reviewer does the functional review on the resulting test build at build]https://test.skohub.io/skohub-io/testing-skohub-vocabs/heads/main/.
+5. If needed, adustments are made in the feature branch and back to 1.).
+6. If the changes are functionally approved, the code review can follow.
+
 ##### Code Review
 
 In code review, the technical implementation of the bugfix or the new feature is reviewed.