Tweak the README

[paulo-ferraz-oliveira]

Prompted by a recent request for help regarding local install of elvis, I got to thinking about the README, and ended up tweaking it for freshness.

I think I properly ported all the prior content, and also:

1. expanded on what elvis is
2. improved the installation instructions
3. documented all the options
4. linked to the elvis_core configuration and rules, where appropriate

⚠️ after this is accepted/merged we should probably publish the documentation to Hex (I don't know if you have to release for it to be linked to "latest").

Edit: this is probably best seen as rich diff.

Edit: I prevented CI via [no ci] on the commit message, but made sure proper markdownlint was done locally. Also, rebar3 ex_doc doesn't complain.

[elbrujohalcon]

💗