Add capabilities dependencies to the readme

[isaiahtaylorhh]

This app introduces at least a few capabilities that need to be added to info.plist and other places on iOS.

• Apple notification service
• Photo library
• Contacts?
• Location?
• Speech Recognition?
• Calendars?

I'm not sure what's generating all these requirements in my app. The top two at least are clearly from this package. Could all of the necessary capabilities be listed?

[imtoori]

@isaiahtaylor There were some instruction in the README, but some information was missing.
Go here to read more about what we require.

This is all due to dependencies like file_picker, image_picker and video_player.