add comment for github actions depth: 0

.github/workflows/ci.yaml

@@ -29,6 +29,7 @@ jobs:
     steps:
       - uses: actions/checkout@v2
         with:
+          # required for setuptools_scm to correctly version: https://github.com/pypa/setuptools_scm/issues/480
           fetch-depth: 0
       - name: Set up Python ${{ matrix.python }}
         uses: actions/setup-python@v2