fixed typos, added a few requirements
Merge branch 'master' of github.com:w3c-webmob/netinfo

* 'master' of github.com:w3c-webmob/netinfo:
  added a bit of text about ios and the download button
  fixed borked url on images
  Attempt to fix images
Added Dropbox, Spotify, and Rdio

Thanks @anselmh!