Port README to 21st century

[Paul Spooren]

Hi,

I created a pull request on GitHub[0] with a reworked and extended README.

Essentially it uses Markdown instead of plain text and embed the new 
logo and handy links to documentation and community resources.

Adrian requested to announce the idea on the lists as it's basically the 
"business card" or OpenWrt, so here we go.

Best,
Paul

[0]: https://github.com/openwrt/openwrt/pull/3217