add documentation on how to extend the showcase
add documentation on how to extend the showcase

rgbkrk/nteract,nteract/nteract,captainsafia/nteract,nteract/nteract,nteract/composition,captainsafia/nteract,nteract/nteract,jdfreder/nteract,rgbkrk/nteract,rgbkrk/nteract,captainsafia/nteract,nteract/nteract,rgbkrk/nteract,jdetle/nteract,nteract/composition,jdetle/nteract,captainsafia/nteract,jdfreder/nteract,nteract/nteract,jdfreder/nteract,jdetle/nteract,jdfreder/nteract,nteract/composition,rgbkrk/nteract,jdetle/nteract