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

[Robin Westberg]

On Mon, 30 Mar 2020 13:10:23 GMT, Erik Helin <ehelin at openjdk.org> wrote:

> 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

Looks good!

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Marked as reviewed by rwestberg (Reviewer).

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