Create PULL_REQUEST_TEMPLATE.md (GH-3404)

Explain that backport PR should include branch name and the original PR number.

diff --git a/.github/PULL_REQUEST_TEMPLATE.md b/.github/PULL_REQUEST_TEMPLATE.md
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+++ b/.github/PULL_REQUEST_TEMPLATE.md
@@ -0,0 +1,10 @@
+!!! If this is a backport PR (PR made against branches other than `master`),
+please ensure that the PR title is in the following format:
+
+[X.Y] <title from the original PR> GH-NNNN
+
+Where: [X.Y] is the branch name, for example [3.6]
+
+GH-NNNN refers to the PR number from `master`.
+
+PLEASE: Remove this headline!!!