Add instructions to readme.md so users can use jupyter lite contents flag

Author: mcbarton

README.md

@@ -101,8 +101,8 @@ To build Jupyter Lite with this kernel without creating a website you can execut
 ```bash
 micromamba create -n xeus-lite-host jupyterlite-core
 micromamba activate xeus-lite-host
-python -m pip install jupyterlite-xeus
-jupyter lite build --XeusAddon.prefix=$PREFIX
+python -m pip install jupyterlite-xeus jupyter_server
+jupyter lite build --XeusAddon.prefix=${{ env.PREFIX }} --contents ../notebooks/xeus-cpp-lite-demo.ipynb --output-dir dist
 ```
 
 We now need to shift necessary files like `xcpp.data` which contains the binary representation of the file(s)