Replace README.txt with a README.md following standard format

Thanks for your work on this @Sonal Gyanani!

I think we need to do some more on this before it can be merged.

I think the Caveats and Assumptions part is better off where it was in the JSON:API Reference section rather than a section on its own.

I don't know where the copy from the Configuration section came from, but it doesn't describe how to configure this module. There is a video that shows how to configure this module at https://youtu.be/MrDHyEkJv3w that would be a good starting point to write some instructions.

Addressed #9. There are still some configuration instructions that needs to be added. Thanks