Add instructions for enabling back gesture on iOS

[graemian]

Most users expect to be able to use the back gesture in apps. My app really felt 2nd-class until I enabled this. I wasn't able to figure out how through a web search. Eventually I asked on the forums and got an answer (https://forums.meteor.com/t/getting-back-gesture-to-work-on-ios/60771)

I really this is a very important feature that should be mentioned here. Perhaps it should even be the default setting?

[CLAassistant]

All committers have signed the CLA.