Docs for check command should include tips for GitHub Actions

[Jesse Attas]

When my repo added the beachball check command to a GitHub Action which runs on PRs, it didn't detect changes correctly. The issue turned out to be the same as the one described in #420.

It would have been helpful if the fix from that issue were described in the documentation for the check command.

My proposal would be to include something like this in a new GitHub Actions section on the check docs.

- uses: actions/checkout@v2
  with: 
    fetch-depth: 0

I don't know if that fix is broadly applicable, but it worked in my repo and I see that Microsoft FAST uses it too.

[Elizabeth Craig]

Thanks, I actually noticed that this was broken in the beachball repo itself too (fixed in #558). 😆

[Kenneth Chau]

I think in github repos, we should create a beachball github action itself that does this check & release for you ultimately...