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

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.