In support of moving to GIT as part of project Skara we need `README.md` and `CONTRIBUTING.md` files at the top of the jfx-dev/rt repo.

These two files already exist in the javafxports/openjdk.jfx GitHub sandbox in the .github directory, which means that the README is displayed on the main repo page here:

https://github.com/javafxports/openjdk-jfx

As part of this fix, we must remove the existing placeholder README file (which was previously removed from the master and develop branches of the javafxports/openjdk.jfx sandbox).

The contents of the two new files will be similar to the existing files in the .github dir of the sandbox, but will differ in some key points. For example, it will indicate that it is the official repo, rather than a sandbox mirror. Eventually, we will add some information about using the Skara tooling.

The .github/README.md file takes precedence over README.md, which allows us to push these changes without disturbing the existing content for the sandbox repo, which will make for a smoother transition. The new README.md (which will point to the new CONTRIBUTING.md file) will be seen when looking at:

https://github.com/openjdk/jfx

In the sandbox, the existing .github/README.md file (which points to the .github/CONTRIBUTING.md file) will supersede the top level README.md file when looking at:

https://github.com/javafxports/openjdk-jfx