README.md: fix repo URL for initial setup

Copy-and-paste error from the time when the
kubernetes-csi/csi-release-tools repo didn't have the code...

initial content

The actual repository was not named like the prototype repo.

It's worth calling out explicitly that only the master branch is
maintained.

The repo was created with an HTML version of the build.make file from
https://github.com/pohly/csi-build-rules/. Here's the raw file.