Add PR template

Signed-off-by: Joffrey F <joffrey@docker.com>

.github/pull_request_template.md

+<!--
+
+Thank you for your interest in contributing to Docker Machine!
+Please note that the project is now in MAINTENANCE MODE, meaning we will
+no longer review or merge PRs that introduce new features, drivers or
+provisioners. We will continue to consider and review proposed bug fixes
+and dependency upgrades when appropriate.
+
+Thank you for your understanding.
+
+-->
+
+## Description
+
+<!-- In a couple sentences, explain what your PR does and what problem it fixes -->
+
+## Related issue(s)
+
+<!-- Include any issue from the tracker that this PR addresses or otherwise relates to -->