README and documentation update

[rosado]

This PR contains changes to update Drafter's README in line with recent changes in muttnik project.

Motivation

Documentation needed a bit of a refresh and better organisation. Additionally, the repo will be made public soon, and what drafter was not easy to grok from existing documentation.

Notes For reviewers

As with the muttnik documentation changes, this PR is supposed to be starting point for a discussion, so any feedback is welcome.

The main additions are:

• the Dependencies section in the README - like in muttnik,
• "What is drafter" document, lifted in full from @RickMoynihan's https://github.com/Swirrl/drafter/pull/619 - I think it was valuable even in an unfinished form (hence marked as draft)

Otherwise it's re-shuffling existing content and removing outdated/conflicting information wherever I spotted it.

Fixes #658

[RickMoynihan]

Thanks @rosado this looks like a big improvement. I've pushed a few changes and made some comments/suggestions - but generally I'm really happy with it.

🙇

[scottlowe]

I rebased against master to include the recent deploy / pack fix for the broken build. This PR is green again ✅

Let's get it merged?