Document how to use url checker for pull request changes

[ge0ffrey]

Presume we build a website with a static website generator (jekyll, jbake, hugo, ...)
The output website is in directory output.

How do we check if there are no new broken url's in a pull request?
After building it from a pull request, by checking the files in the output directory.

It always checks out a new branch, because parameter branch defaults to master, so it ignores the changes in the pull request.
Document how to tell it to use the current branch (don't check anything out).

[vsoch]

We probably just need to make branch optional (and not default to master) so it's not added. Would this work?

[vsoch]

Please test this branch https://github.com/urlstechie/urlchecker-action/tree/test-empty-branch