RFR: 292: Add comment to PR when it becomes outdated

[Robin Westberg]

Hi all,

Please review this change that posts an instructional message on how to sync with the target branch the first time a PR
is marked as outdated.

Best regards,
Robin

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Commit messages:
 - Post an instructional message when a PR is flagged as outdated

Changes: https://git.openjdk.java.net/skara/pull/513/files
 Webrev: https://webrevs.openjdk.java.net/skara/513/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-292
  Stats: 29 lines in 2 files changed: 29 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/skara/pull/513.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/513/head:pull/513

PR: https://git.openjdk.java.net/skara/pull/513