Document best practices on iOS

[peitschie]

Spawned from discussion on #963

The best approaches found on Android are:

1. ~~Use autoConnect and don't explicitly scan (i.e., let the OS schedule this itself)~~ (in iOS, autoConnect is implemented purely in the JS layer, and is identical to approach 2)
2. Explicitly scan and then use connect
3. (Maybe) for a non-bonded device, it's safe to just use connect without an explicit scan (to be confirmed)

Additional things worth documenting:

1. Handling Bluetooth authorization
2. Known quirks regarding isEnabled state
3. Background processing vs Restore state

Documenting the known caveats and quirks would help implementors more effectively use this plugin.