Update README about minimum iOS version

[pnthach95]

I hit this error when pod install

[!] CocoaPods could not find compatible versions for pod "react-native-bottom-tabs":
  In Podfile:
    react-native-bottom-tabs (from `../node_modules/react-native-bottom-tabs`)

Specs satisfying the `react-native-bottom-tabs (from `../node_modules/react-native-bottom-tabs`)` dependency were found, but they required a higher minimum deployment target.

So I update README about minimum iOS version.

[okwasniewski]

Hey!

Thanks for opening the PR. Could you also mention how to fix this issue for Expo?

Here is an open issue instructing how to achieve this: #18

[pnthach95]

Hey @okwasniewski, I added Expo part to README

[okwasniewski]

Thanks for this!