docs: add a GitHub pull request template

Signed-off-by: Joakim Bech <joakim.bech@linaro.org>
Acked-by: Jerome Forissier <jerome.forissier@linaro.org>

.github/pull_request_template.md

@@ -0,0 +1,19 @@
+<!--
+    If you are new to submitting pull requests to OP-TEE, then please have a
+    look at the list below and tick them off before submitting the pull request.
+
+    1. Read our contribution guidelines:
+         documentation/github.md.
+
+    2. Read the contribution section in Notice.md and pay extra attention to the
+       "Developer Certificate of Origin" part:
+         https://github.com/OP-TEE/optee_os/blob/master/Notice.md#contributions.
+
+    3. You should run checkpatch preferably before submitting the pull request.
+
+    4. When everything has been reviewed, you will need to squash, rebase and
+       add tags like `Reviewed-by`, `Acked-by`, `Tested-by` etc.
+
+    NOTE: This comment will not be shown in the pull request, so no harm keeping
+    it, but feel free to remove it if you like.
+-->