Update readme with new parameter

Author: luispabon

README.md

@@ -29,6 +29,7 @@ A branch must meet all the following criteria to be deemed abandoned and safe to
 | `last_commit_age_days` | How old in days must be the last commit into the branch for the branch to be deleted. Default: `60` | `90`
 | `dry_run`* | Whether we're actually deleting branches at all. Possible values: `yes, no` (case sensitive). Default: `yes` | `no`
 | `github_token`* | The github token to use on requests to the github api. You can use the one github actions provide | `${{ github.token }}`
+| `github_base_url` | The github API's base url. You only need to override this when using Github Enterprise on a different domain. Default: `https://api.github.com` | `https://github.mycompany.com/api/v3`
 
 ### Note: dry run