feat(subscriptions): add option 'proration_behavior.always_invoice' for updates

[arnaldop]

Also adds requirements.txt to make building easier and updates README.

[H--o-l]

Hey @arnaldop, thanks for contributing!

About the requirements.txt I'm not a fan because setup.py describe them well already. Do a pip3 install --user -e . and you'd be done with it.
Also, it's common practice so I'm not sure it should ends-up in the README.

Could you reduce the title length for the second commit so it fits in a GitHub line (see the ... in the screen below)?

Last thing, could you add an explanation in the commit message about what this change is, what it does, links to the real Stripe API to show us where it comes from, etc.
We don't know Stripe API by heart, and we will have to maintain the change you propose in the long term, so can you advocate your change to us?

The last thing, did adding a test would make sense? If not can you explain why? If yes could you add a test?

Thanks in advance!