doc: use consistent markdown in README

Improve markdown practices in README:

* consistent header indicators
* consistent emphasis indicators
* wrap bare URLs in `<` and `>`
* wrap at 80 chars
* specifying language in fenced code

PR-URL: #7971
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>