DOC: add documentation on app usage to the output README.md

[khiron]

Update the cookiecutter output README.md, which is the readme that will appear at the root of the expanded project - as opposed to the readme for the app_template project, which is how to run cookiecutter.

We need to;

• give some examples for how to call an app
• whether the dev should make an app composable
• naming conventions for apps and how to resolve collisions

[GavinHuttley]

I need to expand the description on the codent3.org site and then just link to that in this template.

This is the page that needs to be modified. Will do that first.

[GavinHuttley]

I think just add a link to https://cogent3.org/doc/app/user_function.html in the read me is sufficient.