Add type hints documentation

[camilamaia]

Feature request

Description the feature

We are introducing type hints to the project: #431 and for that we need a clear documentation of:

• How to use type hints
• When to use type hints

Also, to make it clear this is optional:

• "If you're not familiar with typing then just forget about it and write usual python code"
• "If you're familiar with typing - new type annotations are very welcome"

[taconi]

Perhaps pep 484 is good documentation on type hinting, and as the application is a CLI, it may not be necessary to inform in the project documentation how to use type annotations.
About adding the input/output types of the functions I could be doing this