Update Docs and Re-publish

[nelsonic]

The code in this package/repo works. ✅ 🎉
But there is nowhere near enough documentation for someone else to use the package without reading the code in /lib ... as a result only we (@dwyl) are using it and nobody else is benefiting from it.

We need to comprehensively document the package (a) because it makes it easier to maintain, (b) it makes it easier (possible) for other people to use it and get their GitHub Auth enabled projects off the ground and (c) other people in the elixir/dwyl community can contribute improvements.
Similar to dwyl/elixir-auth-google#10 and PR: dwyl/elixir-auth-google#11

Thankfully this package's /lib/elixir_auth_github.ex already has pretty decent @doc comments 👍
So all that is missing is are the detailed instructions for getting it setup.

Compare the difference:
before: https://github.com/dwyl/elixir-auth-google/tree/2b1f9b09433fb7845dff00f593edd8fa29825f5f
after: https://github.com/dwyl/elixir-auth-google/tree/71c022afee27e2cc377936c686538afa8e08d13e

Todo

• add GitHub credentials instructions similar to https://github.com/dwyl/hapi-auth-github/blob/master/GITHUB-APP-STEP-BY-STEP-GUIDE.md

  https://github.com/settings/applications/1190344

• Create "Sign in with GitHub" button in CSS & SVG Sign in with GitHub Button #33
• Create a demo app similar to https://github.com/dwyl/elixir-auth-google-demo
  • Deploy the demo app to Heroku so people can test it.

  https://dashboard.heroku.com/apps/elixir-auth-github-demo/settings

• extend the README.md with comprehensive instructions for adding elixir-auth-github to a sample Phoenix App.
• Re-publish the hex.pm package
• update any @dwyl apps that consume this package.

before: https://github.com/dwyl/elixir-auth-github/tree/44021233add025348f261f5bf0b9b32fe6c5711a
after: 🔜 https://github.com/dwyl/elixir-auth-github/tree/update-docs-issue%2332

[nelsonic]

Continue: https://github.com/dwyl/elixir-auth-github-demo#-continue-here