[13] RFR: JDK-8223719: Add .gitignore to repo

[Kevin Rushforth]

Simple fix to add a .gitignore file to the upstream HG 
openjfx/jfx-dev/rt directory. This file currently exists as a 
"javafxports-only" file in the javafxports/openjdk-jfx sandbox on GitHub.

https://bugs.openjdk.java.net/browse/JDK-8223719
http://cr.openjdk.java.net/~kcr/8223719/webrev.00/

This reduces the number of files that are unique to the 
javafxports/openjdk-jfx GitHub sandbox, and also allows it to show up on 
the jfx-dev/rt mirror that Project Skara has created [1]

-- Kevin

[1] https://github.com/openjdk/jfx