Add pull request template discouraging usage of pull requests

since we use Gerrit for code review.

Change-Id: I2eeea82452d678564e17e3db9bd6ca7ef4ae4cc2

Author: msohn

.github/pull_request_template.md

@@ -0,0 +1,5 @@
+Thank you for contributing to JGit!
+
+JGit uses [GerritHub](https://eclipse.gerrithub.io) for code changes and review, therefore **pull requests in this repository cannot be merged**.
+
+Please make sure you have read the section on [contributing patches](https://github.com/eclipse-egit/egit/wiki/Contributor-Guide#contributing-patches) of the [Contributor Guide](https://github.com/eclipse-egit/egit/wiki/Contributor-Guide).