Update README.md

[jacegu]

We are:

• Updating the structure of the file
• Incorporating a Requirements section to reflect the supported Ruby versions.
• Incorporating an explicit Documentation section, that includes:
  • Links all of the available sources of documentation.
  • A better structured example section (that we will be slowly growing).

There are no significant content changes, other than the addition of the documentation links. Everything else is just restructuring.

[nestorsalceda]

It's an improvement. Thank you for this!