hg: openjfx/jfx-dev/rt: 8227540: Add README.md and CONTRIBUTING.md to jfx repo

[kevin.rushforth at oracle.com]

Changeset: 7141b71a44ba
Author:    kcr
Date:      2019-07-17 08:30 -0700
URL:       https://hg.openjdk.java.net/openjfx/jfx-dev/rt/rev/7141b71a44ba

8227540: Add README.md and CONTRIBUTING.md to jfx repo
Reviewed-by: prr, arapte, jvos

+ CONTRIBUTING.md
+ README.md