Add podspec

[jpaas]

Fixes #2 for the people who use cocoapods with RN

[cjpete]

Would be good to also update the README with the cocoapods setup:

pod 'react-native-open-settings', :path => '../node_modules/react-native-open-settings'

Edit: I'm using this fork in my project, it would be great to get this merged:

  ...
  "react-native-open-settings": "jpaas/react-native-open-settings#add-podspec",