Explain which branch to select for Pull Requests

Author: javiereguiluz

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,9 @@
+<!--
+
+If your pull request fixes a BUG, use the oldest maintained branch that contains
+the bug (see https://symfony.com/roadmap for the list of maintained branches).
+
+If your pull request documents a NEW FEATURE, use the same Symfony branch where
+the feature was introduced (and `master` for features of unreleased versions).
+
+-->