Generate docs using the README.md

[vermaxik]

Hello 👋

I was found this package through the hexdocs and was confused about instruction how to use it.

Later I realized that you have a great README section with all the needed information.

To make it up to date in both places, I would like to propose to generate ex_doc from README.md

In addition, I bumped also the version to the latest one 3.0.13 and changed it everywhere.

[stefanchrobot]

👍 on this one as the docs on hexdocs are far from perfect. The README.md as where people will find all the information.

[josevalim]

Closing this for now. Feel free to reopen if updated. :)