github: Add PR template

Adding a link to related part of compose spec is much easier for the
contributors, because they're already working on the feature. This saves
reviewer time.

Signed-off-by: Povilas Kanapickas <povilas@radix.lt>

Author: p12tic

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,7 @@
+
+## Contributor Checklist:
+
+If this PR adds a new feature that improves compatibility with docker-compose, please add a link
+to the exact part of compose spec that the PR touches.
+
+All changes require additional unit tests.