RFR: skara: add hint about help for commands

[Erik Helin]

Hi all,

please review this small patch that adds information on how to show more help
for individual commands to `git skara help`.

Testing:
- Manually running `git skara help`

Thanks,
Erik

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Commit messages:
 - skara: add hint about help for commands

Changes: https://git.openjdk.java.net/skara/pull/596/files
 Webrev: https://webrevs.openjdk.java.net/skara/596/webrev.00
  Stats: 4 lines in 1 file changed: 4 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/skara/pull/596.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/596/head:pull/596

PR: https://git.openjdk.java.net/skara/pull/596