Tidy up the README
Tidy up the README

Improve the installation instructions somewhat and organize the text
a bit better.

jonmartindell/rmagick,jonmartindell/rmagick,rmagick/rmagick,btakita/rmagick,btakita/rmagick,jonmartindell/rmagick,carsonreinke/rmagick,mockdeep/rmagick,tvon/rmagick,prognostikos/rmagick,mockdeep/rmagick,antonifs/rmagick,btakita/rmagick,carsonreinke/rmagick,tvon/rmagick,mockdeep/rmagick,prognostikos/rmagick,antonifs/rmagick,tvon/rmagick,rmagick/rmagick,mockdeep/rmagick,antonifs/rmagick,prognostikos/rmagick,carsonreinke/rmagick,rmagick/rmagick