Fix cljfmt fix on PR review, update README with more examples

[SerhiiKozachenko]

Fixes cljfmt fix on PR review, problems were:

• On pull_requestthe env var GITHUB_REF points to PR merge branch refs/pull/:prNumber/merge and _remote_commit fails, fixed by using GITHUB_HEAD_REF.
• On pull_request_review these are empty, and we need to parse PR head ref from action event and pass it via new env var GITHUB_PR_HEAD_REF, added as an example to README.