[PULL_REQUEST_TEMPLATE.md] Initial commit

- Add PULL_REQUEST_TEMPLATE.md file to direct contributors away
  from GitHub pull requests and to our official contribution
  process

Change-Id: I58fcbebb9439a2799f31a563ce30c429db93893c
diff --git a/PULL_REQUEST_TEMPLATE.md b/PULL_REQUEST_TEMPLATE.md
new file mode 100644
index 0000000..6a67cb6
--- /dev/null
+++ b/PULL_REQUEST_TEMPLATE.md
@@ -0,0 +1,5 @@
+Welcome, and thank you for your contribution!
+
+We don't use GitHub pull requests for contribution in Mundane. Please see
+[`CONTRIBUTING.md`](https://github.com/google/mundane/blob/master/CONTRIBUTING.md)
+for contributing instructions.
\ No newline at end of file