Add branch information to the pull request template

Author: soyuka

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,3 +1,4 @@
+<!-- Please update this template with something that matches your PR -->
 | Q             | A
 | ------------- | ---
 | Bug fix?      | yes/no
@@ -9,4 +10,4 @@
 | License       | MIT
 | Doc PR        | api-platform/doc#1234
 
-*Please update this template with something that matches your PR*
+<!-- Bug fixes should be based against the current stable version branch, master is for new features only -->