Properly document requestPermissions

[cfjedimaster]

As the title says - the docs in the readme do not specify that requestPermissions returns a promise. I lost quite a bit of time trying out figure this out. :\

And here is an example of someone else also having an issue: https://stackoverflow.com/questions/51190059/camera-capture-function-is-not-working-for-ios

[lini]

Apologies for the confusion. The docs will be updated to reflect this.