minor #9405 Explain which branch to select for Pull Requests (javiereguiluz)

This PR was squashed before being merged into the 2.7 branch (closes #9405).

Discussion
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Explain which branch to select for Pull Requests

I'm very tired of explaining the same again and again ... and also tired of fixing the branch "errors" (they are not errors because we don't explain this anywhere on GitHub; hence this proposed PR).

Commits
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348f715 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).
+
+-->