workflows: add available labels 'long-term'
add 'long-term' to the workflows readme.
| ok-to-merge | run mergebot and merge (rebase) current PR |
| ci/integration-docker-ok | integration test is able to build docker image |
| ci/integration-gcp-ok | integration test is able to run on GCP |
+| long-term | long running pull request, don't close |
