RFR: 846: Document the need to enable Github Actions on personal forks that offer testing

[Robin Westberg]

Forks created after pre-submit testing using GitHub actions was introduced need to explicitly allow GitHub actions. Post a notice as a separate check result if we detect this condition.

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Commit messages:
 - Initial implementation

Changes: https://git.openjdk.java.net/skara/pull/974/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=974&range=00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-846
  Stats: 135 lines in 10 files changed: 84 ins; 29 del; 22 mod
  Patch: https://git.openjdk.java.net/skara/pull/974.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/974/head:pull/974

PR: https://git.openjdk.java.net/skara/pull/974