Adds GitHub pull request template

This template advices potential contributors to upload their CLs to
Gerrit instead of creating a pull request in GitHub.

Change-Id: Ibeb1a65229bb2ee505915ee728a176ca19feb14c
diff --git a/.github/pull_request_template.md b/.github/pull_request_template.md
new file mode 100644
index 0000000..40b3209
--- /dev/null
+++ b/.github/pull_request_template.md
@@ -0,0 +1,16 @@
+<!--
+  ⚠️ IMPORTANT
+
+  Hi there! Thanks for contributing!
+
+  This project uses Gerrit for code review. Please, submit your pull
+  request there so we can review it:
+  * https://gerrit.avm99963.com/
+
+  You can learn how to do that in the following document:
+  * https://gerrit.avm99963.com/Documentation/user-upload.html
+
+  If you're not comfortable using Gerrit, you can submit your pull
+  request here at GitHub and we'll manually transfer it to Gerrit :)
+-->
+