Readme: remark on how to build cachedly

Signed-off-by: Marcus Müller <mueller@baseband.digital>

Author: Marcus Müller

README.md

@@ -6,3 +6,12 @@ This repository, the GNU Radio project keeps the docker containers for the CI th
 The organization is such that you find the individual containers in directories below ci/, generally named `distro-distro version-GNU Radio version`.
 
 For the containers that get re-built on PRs and commits to this repo itself, see .github/workflows/build-dockers.yml . When making a PR for a new worker, don't forget to include that in that file!
+
+Making PRs
+----------
+
+If you have push access to unprotected branches on this repo, feel encouraged
+to push your branch as `username/branchname` to the gnuradio/gnuradio-docker
+and make a PR from there. This has the advantage that you get to use our docker
+layer/artifact caching, which means that unchanged steps Dockerfiles build
+within seconds instead of minutes.