Update CONTRIBUTING.md to point to GitHub issues

With a link to older bugs in Bugzilla.
I have no idea if this is an accurate description of the situation,
it's all guesswork on my part.

Signed-off-by: Antonin Delpeuch <antonin@delpeuch.eu>
Change-Id: I019689115a744daf588a86b44e998d2dfe03d4b5

Author: wetneb

CONTRIBUTING.md

@@ -8,7 +8,9 @@ Contributions require that you sign the [Eclipse Contributor Agreement](https://
 For anything other than small changes, it's a good idea to open a bug
 report for it (in case one doesn't already exist). This gives others the
 chance to give input and is useful for tracking. 
-[Create JGit bugs here](https://bugs.eclipse.org/bugs/enter_bug.cgi?product=JGit).
+[Open JGit issues on GitHub](https://github.com/eclipse-jgit/jgit/issues).
+
+Older bugs can be found [in Bugzilla](https://bugs.eclipse.org/bugs/buglist.cgi?bug_status=__open__&component=JGit&list_id=21379030&product=JGit).
 
 ## Submitting changes