Add documentation for options including `publishResultAs`, `scmVars`, etc

[codeocelot]

Hi maintainers!

First, thanks for publishing this plugin. It's been a great help to my team's workflow.

While using this plugin, I found the README documentation pretty unhelpful because it didn't explain the available options for the plugin. In fact, the documentation does not produce the expected results in at least one case-- publishResultsAs is always defined as statusCheck throughout the README, but the screenshot at the top of the docs corresponds to publishResultsAs: 'comment'.

Additionally, scmVars is undocumented and it's not intuitive that GIT_BRANCH, GIT_COMMIT need to be provided to discover the PR.

It may be helpful to add further documentation beyond this, but these are the pain-points I discovered and only resolved by diving into the plugin source (and after many failed Jenkins jobs!)

As to a resolution, I'm happy to submit a PR to add these sections to the README if the maintainers are in support of that. Just let me know.

[dmotpan]

hey @codeocelot
Feel free to submit a PR. Thank you!