Use Markdown syntax rather than HTML for image links.
Use Markdown syntax rather than HTML for image links.

At some point pub.dartlang.org will support literal <a> and <img> tags, but
using Markdown links will get the readme displayed correctly for now.

threeDart/three.dart,brason/three.dart,james-storey/three.dart,hirosuke999/three.dart,brason/three.dart,threeDart/three.dart,hirosuke999/three.dart,james-storey/three.dart,hirosuke999/three.dart,brason/three.dart,threeDart/three.dart