Add a GitHub pull request template to discourage PRs there.

.github/pull_request_template.md

@@ -0,0 +1,8 @@
+## Hold up!
+
+Thanks for your contribution! Unfortunately, we don't use GitHub pull requests
+to manage code contributions to this repository (and GitHub doesn't have any
+way to disable pull requests entirely). Instead, please see
+https://torsion.org/borgmatic/#contributing which provides full instructions
+on how to submit pull requests. You can even use your GitHub account to login.
+