Add documentation for invoke

[briggySmalls]

I really like using invoke for linting/formatting/testing etc. but people may not realise how to use it.

This should be added to the project documentation

[yngtodd]

I am curious about this. Currently when I run invoke --release on a new project, I am running into a missing file error for .coverage:

FileNotFoundError: [Errno 2] No such file or directory: '/Users/ygx/src/rust/ferris/.coverage'

Are there extra args for invoke coverage that create this file, or does it need to be created manually?

[briggySmalls]

@yngtodd thanks so much for your message. You found a bug 🙈

I've just pushed a fix, so future projects should not have this issue.

The problem is in the clean_tests function of tasks.py. The code will throw errors if the directories it is cleaning are not present.

You can see the fix I pushed in c6ac555. If you are using python 3.8 then you can just add the missing_ok=True parameter to the call to unlink. But I've pushed a fix that supports all versions of python.

Sorry for the confusion, and thanks for using the tool