GitHub Releases: document `repo` option to upload release to a different repository

[iriberri]

This is possible by using the repo option: https://github.com/travis-ci/dpl/blob/327540f04ec4dc7d642c3285b7f7bc305e2cb8c1/lib/dpl/provider/releases.rb#L39-L41.

It is not currently documented in the GitHub Releases page, so I guess it should be :)

As a note, this option can be confused with the on.repo one for the conditional deployments, so it'd be nice to add a note about that.

The GitHub Pages deployment does also have this feature (https://github.com/travis-ci/dpl/blob/327540f04ec4dc7d642c3285b7f7bc305e2cb8c1/lib/dpl/provider/pages.rb#L9), which is documented but not in much detail. (github_url should not be edited, and the repo should contain the whole slug - this has been a cause of confusion in the past)

[stale[bot]]

Thanks for contributing to this issue. As it has been 90 days since the last activity, we are automatically closing the issue. This is often because the request was already solved in some way and it just wasn't updated or it's no longer applicable. If that's not the case, please do feel free to either reopen this issue or open a new one. We'll gladly take a look again! You can read more here: https://blog.travis-ci.com/2018-03-09-closing-old-issues

[Anmolreshi]

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