Create PULL_REQUEST_TEMPLATE.md (GH-3404)

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

.github/PULL_REQUEST_TEMPLATE.md

+!!! 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!!!