Rename readme to caps

Gemspec wants README.md producing this message:

pose at /home/me/.rbenv/versions/2.0.0-p0/lib/ruby/gems/2.0.0/bundler/gems/pose-aa83cde5ad24 did not have a valid gemspec.
This prevents bundler from installing bins or native extensions, but that may not affect its functionality.
The validation message from Rubygems was:
  ["README.md"] are not files