RFR: 339: Only show link to CONTRIBUTING.md if it exists

[Erik Helin]

Hi all,

please review this patch that ensures that the pull request only shows a link to
a `CONTRIBUTING.md` file in the integration message if a `CONTRIBUTING.md`
actually exists.

Testing:
- Added two new unit tests
- `make test` passes on Linux x64

Thanks,
Erik

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Commit messages:
 - SKARA-339

Changes: https://git.openjdk.java.net/skara/pull/551/files
 Webrev: https://webrevs.openjdk.java.net/skara/551/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-339
  Stats: 95 lines in 2 files changed: 89 ins; 0 del; 6 mod
  Patch: https://git.openjdk.java.net/skara/pull/551.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/551/head:pull/551

PR: https://git.openjdk.java.net/skara/pull/551