README.md - intstall from  NPM

Dear friend, I'm using couchapp for 3 years now, and love it.

I think after so long the project is mature enough to replace the **installation** section in the README.md to

  `npm install couchapp -g`

And if you _really_ want to be nice you can give a disclaimer for windows users, their npm generated cmd files and the PATH they should assure configured.

:)
