Request to backport 4 fixes to 11-dev + add README files in support of GIT transition

[Kevin Rushforth]

Hi Johan,

I request approval to backport the following fixes to 11-dev in support 
of the transition to GIT:

8231735: gradle checkrepo is obsolete and doesn't work with git
8223719: Add .gitignore to repo
8223722: Cleanup .gitignore file
8227920: Remove dummy README from FX repo

And also to create the needed README files using the following bug; 
since this one isn't  backport of existing content, I also provided a 
draft PR for review purposes:

8265667 : Add README.md and CONTRIBUTING.md to FX 11-dev/rt repo
https://github.com/kevinrushforth/jfx/pull/2
https://bugs.openjdk.java.net/browse/JDK-8265667

-- Kevin