Remove README in favor of README.md

Complete the transition to README.md I started 3 years ago. Remove the
now-redundant README file. It's currently just README.md w/o the light markup
and adds no real value. This also allows us to use additional MarkDown
markup as we see fit w/o worrying about keeping things in sync.

(cherry picked from commit 8a157722e947191a63e9108cbfb60ee2605858b4)