Update README.md

Author: josecelano

README.md

@@ -9,6 +9,7 @@ This repo contains some Python examples on using git and GitHub API.
 1. [Sign automatic commits in GitHub Actions](./docs/how_to_sign_automatic_commits_in_github_actions.md)
 2. [How to create a single commit with multiple files using GitHub API](./docs/how_to_create_a_single_commit_with_multiple_files_using_github_api.md)
 3. [How to sign commits using the GitPython package](./docs/how_to_sign_commits_using_the_gitpython_package.md)
+4. [How to create a subkey for signing](./docs/how_to_create_a_subkey_for_signing.md)
 
 ## Build and run