Overhaul main README

[johnbaldwin]

The current README is horribly out of date and it is also serving as the changelog file, eating valuable reading real estate.

Tasks to overhaul README

• Update CHANGELOG and remove changelog content from README.md
• Structure the doc (see doc structure below)
• Include content: Document Figures and Open edX versions compatibility table #436

WIP Doc structure

• Overview: This section is the 30 second read. What's Figures? Is it active? Is it useful to me? Do I continue reading?

• What is Figures? After the 30 second read, provide some more details, still brief. We ought to have architecture doc(s) and link from here

• Getting started with Figures

• Figures devsite

• Installing Figures in Open edX

• Contributing

• Reporting bugs

• reporting security issues