If you'd like to contribute example applications, you'll likely use create-react-app. Unfortunately, you won't be able to initialize your example by running create-react-app myexample directly in this directory. This is because create-react-app does not work yet with yarn workspaces and lerna. There is a workaround though:
Initialize your new example application outside react-admin folder then simply move the newly created folder inside the examples folder. Finally, run yarn at the react-admin root folder.
Tip: Ensure you don't commit a yarn.lock inside your new example application folder.