Add Readme

Thanks user  for pointing out the need for this.

Signed-off-by: Marcus Müller <[email protected]>

README.md

@@ -0,0 +1,8 @@
+GNU Radio Docker Containers
+---------------------------
+
+This repository, the GNU Radio project keeps the docker containers for the CI that builds GNU Radio and runs tests, every time someone makes a pull request against the GNU Radio code base.
+
+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!