RFR: 8251552: Add minimal CONTRIBUTING.md file

[Erik Helin]

Hi all,

please review this patch that adds a minimal CONTRIBUTING.md file to the 
repository. Many source code forges (e.g. GitLab, GitHub, BitBucket) 
show the CONTRIBUTING.md file when a newcomer makes their first 
contribution. Since we already have an OpenJDK developer's guide [0], 
I've chosen to go with a minimal CONTRIBUTING.md file that only refers 
the reader to the developer's guide.

Issue:
https://bugs.openjdk.java.net/browse/JDK-8251552

Webrev:
https://cr.openjdk.java.net/~ehelin/8251552/00/

Thanks,
Erik

[0]: https://openjdk.java.net/guide/