Add GitHub PR template to point contributors at CONTRIBUTING.rst

GitHub's link to the contribution guidelines in `CONTRIBUTING.rst`
is not prominent enough to be reliably noticed and followed by
new contributors.  Add a PR template to make the link more visible.

.github/pull_request_template.md

+Thanks for your interest in contributing to CMake!  The GitHub repository
+is a mirror provided for convenience, but CMake does not use GitHub pull
+requests for contribution.  Please see
+
+  https://gitlab.kitware.com/cmake/cmake/tree/master/CONTRIBUTING.rst
+
+for contribution instructions.  GitHub OAuth may be used to sign in.