RFR: GitHub Actions

[Jesper Wilhelmsson]

Encouraging developers to enable GHA on their personal forks.

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Commit messages:
 - GitHub Actions

Changes: https://git.openjdk.java.net/guide/pull/80/files
 Webrev: https://webrevs.openjdk.java.net/?repo=guide&pr=80&range=00
  Stats: 8 lines in 1 file changed: 8 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/guide/pull/80.diff
  Fetch: git fetch https://git.openjdk.java.net/guide pull/80/head:pull/80

PR: https://git.openjdk.java.net/guide/pull/80