RFR: 8231326: Update README and CONTRIBUTING docs for Skara

[Kevin Rushforth]

Please review the following to update the top-level README.md and 
CONTRIBUTING.md files for the switch to Skara:

https://bugs.openjdk.java.net/browse/JDK-8231326
https://github.com/javafxports/openjdk-jfx/pull/601
-- Kevin