[PRI][WIP] add default readme for app template

:koala::+1:

This PR intends to insert a default README.md file in the app template.
<h2>TASKS</h2>
@svizzari , @maximeprades, and @tmcinerney  can you please review this format? I am happy to further refine it if there is any error :)

/cc @zendesk/quokka