Document use of commitlint as a github action (possible own CLI in the future)

Expected Behavior

Provide a github actions cli just like commitlint-travis.

Context

It should easy setup for running commitlint as part of a github actions setup.

[arznaar]

I think you can also just call @commitlint/cli for this and get number of commits from PR event commitlint --from HEAD~${{ github.event.pull_request.commits }} --to HEAD and fetch all commits in checkout fetch-depth: 0. Here is full example:

on: [pull_request]
jobs:
  lint-commits:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v2.3.1
        with:
          # we actually need "github.event.pull_request.commits + 1" commit
          fetch-depth: 0
      - uses: actions/setup-node@v2.1.0
      # or just "yarn" if you depend on "@commitlint/cli" already
      - run: yarn add @commitlint/cli
      - run: yarn run commitlint --from HEAD~${{ github.event.pull_request.commits }} --to HEAD

[wagoid]

I’ve created a github action for this that doesn’t require any setup, if you want to have a look: https://github.com/marketplace/actions/commit-linter

I tried using @bennypowers’s action but got some issues and ended up creating another one, as explained here

I am already using in some projects, feel free to use it too and let me know if anything goes wrong 😃