[PR] Adding concepts clarification and tutorial

[LuchoTurtle]

closes #1

This PR should be the initial template for the project.
Has a section describing concepts for better understanding and a tutorial to follow through integration with Stripe.

[LuchoTurtle]

Even though it's still in progress, @nelsonic you can take a look at the "theory" part and check if there's anything big missing in it, before I start doing an example.
I didn't think going over the features of each alternative would be worthwhile from an engineering standpoint, as that is just marketing, imo.

[LuchoTurtle]

Added Stripe integration. It should be fully working, with authentication and checkout sessions.
I still want to expand the example more, though. Even though this works, I still want to block user access of unpaid users. Still need to see how that can be implemented with Stripe's API 🤔

[nelsonic]

Sounds good. keep going. looking forward to reading your research findings. 👌

[LuchoTurtle]

This should be done. I've added the project, used DETS for storage (no need for PostgreSQL here) and I got it running with 100% coverage.

Fixed some README typos and changed the Fork example you mentioned in the feedback.

I also added CI (but it didn't execute in this PR).

[nelsonic]

[nelsonic]

Please don't do this. [over] and [over] and [over] is not good link text. dwyl/contributing#119
I understand that you are emphasising the fact that many people have had these same questions.
But as a document that we want to read, maintain and have others contribute to.
We want to set high standards for link text.
Please/thanks! 🙏

[nelsonic]

** (Mix) Expected one or more files/patterns to be given to mix format or for a .formatter.exs file to exist with an :inputs or :subdirectories key

https://github.com/dwyl/learn-payment-processing/actions/runs/3828017445/jobs/6513143646

[nelsonic]

https://github.com/dwyl/learn-payment-processing/actions/runs/3901637053/jobs/6663723897#step:6:152

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (main@da457e4). Click here to learn what that means.
The diff coverage is n/a.

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[nelsonic]

@LuchoTurtle this write-up is superb. 😍 Well done. 🎉

[nelsonic]

Note: I made a few tweaks but your core content is excellent. Keep up the great work. 🚀