docs: add GH_USERNAME input param description (#111)

Author: tuunit

README.md

@@ -51,6 +51,7 @@ Use the following `input params` to customize it for your use case:-
 
 | Input Param    | Default Value                                         | Description                                               |
 | -------------- | ----------------------------------------------------- | --------------------------------------------------------- |
+| `GH_USERNAME`  | Your GitHub username                                  | Username for which to generate the activity overview      |
 | `COMMIT_NAME`  | github-actions[bot]                                   | Name of the committer                                     |
 | `COMMIT_EMAIL` | 41898282+github-actions[bot]@users.noreply.github.com | Email of the committer                                    |
 | `COMMIT_MSG`   | :zap: Update README with the recent activity          | Commit message used while committing to the repo          |