docs: switch help msg away from README?

[jo3-l]

Summary Currently, the help message is being generated from the README: https://github.com/SWC-Python-Club/python-dl/blob/48909716618a5bcb93ccb95ad250561fcf38f280/source/main.py#L10-L14

This is not a problem currently as the README is pretty much a help message at this point but it's unlikely it'll stay like this forever, the README is going to grow as the codebase grows (contributing instructions, license information, images, badges, etc.) and some content may be unsuitable to show for the end user.

Proposed solution Some possible thoughts:

• hardcode the help message as a constant somewhere (not the greatest, as it has to be updated as the program is updated)
• use a cli generator/framework that generates the message dynamically from the code (could be custom-written or some external library, not too familiar with the Python ecosystem so don't have too many thoughts on this)

[AndreiSva]

Indeed I debated wether to use the README as the help message for python-dl, haven't used any cli generators at all but I will definitely look into that and possible change it in the next few commits. Thanks for the feedback!

[jo3-l]

The builtin argparse module might be something to look into - from a very brief peek it seems it generates a help message without too much additional work / additional plugins.