Clarify Python autosubmission prerequisites

Added note about .python-version file requirement for Python autosubmission.

Author: ljones140

content/code-security/supply-chain-security/understanding-your-software-supply-chain/configuring-automatic-dependency-submission-for-your-repository.md

@@ -104,6 +104,8 @@ The .NET autosubmission action uses the open source [component-detection](https:
 ### Python projects
 
 Python uses the open source [component-detection](https://github.com/microsoft/component-detection/) project as its underlying graph generation engine. The autosubmission action for Python will only run if there is a `requirements.txt` file in the root directory of the repository. Python autosubmission does not currently support private packages; packages referenced in `requirements.txt` which are not publicly available will cause the autosubmission action to fail.
+**Note:** This action uses actions/setup-python to install Python. Therefore, you must include a .python-version file in your repository to specify the Python version to be installed.
+
 
 ## Further reading