Add note of how to run this inside a CI

[danilofuchs]

I configured my Github Actions pull_request job to run ts-strictify. The actions/checkout@v2 by default does not fetch all repository refs, so master is not available, leading to this issue:

fatal: ambiguous argument 'master...': unknown revision or path not in the working tree.

I fixed it by using fetch-depth: 0 (will fetch the master branch) and setting the flag targetBranch origin/master.

I think it would be nice to add a note on readme saying that the environment needs to have valid git refs present (which CodePipeline, for instance, does not provide by default) and that pointing to origin/master leads to better results.

[afberg]

If anyone else comes across the same issue:

For some reason it didn't work as an npm script, but moving it directly to the workflow did the trick:

      - name: Checkout
        uses: actions/checkout@v2
        with:
          fetch-depth: 0
       ...
      - name: Strictify
        run: npx ts-strictify --targetBranch=origin/master