Add documentation for accessing private repo (#104)

* Add documentation for accessing private repo

I've tried accessing private repos. Supplied GITHUB_TOKEN does not work. You will need Personal Access Token with repo scope

Co-authored-by: Dawid Dziurla <dawidd0811@gmail.com>

README.md

@@ -16,6 +16,7 @@ Let's suppose you have a workflow with a job in it that at the end uploads an ar
   with:
     # Optional, GitHub token, a Personal Access Token with `public_repo` scope if needed
     # Required, if artifact is from a different repo
+    # Required, if repo is private a Personal Access Token with `repo` scope is needed
     github_token: ${{secrets.GITHUB_TOKEN}}
     # Required, workflow file name or ID
     workflow: workflow_name.yml