README does not explain how to create group menu

The missing steps in the documentation are:
- Browse to a group page
- Click the "All Entities" tab
- Click the "+ Add new entity" button
- Click "Group Menu ({organization-name})"
- Fill in a Title
- Click the "Save" button

Attached a patch that updates the documentation with my steps above.

It's actually the group menu type name in the parentheses; updated the steps to match.