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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91d942e Explain which branch to select for Pull Requests

Author: javiereguiluz