Corrected README to reflect that 'repo_name' is a required parameter.

/hook/ requires the 'repo_name' parameter. Failure to include it results in a 'no repo name' status:

```python
@app.route('/hook/<pull_request_id>/<path:object_key>')
def hook_view(pull_request_id, object_key):
  repo_name = request.args.get('repo_name')
  if not repo_name:
    return jsonify({'status': 'no repo name'})
...
```