Update google_atlas README.md with steps required for iOS

[ezmeev]

As a consumer of google_atlas maps provider for iOS app, I'd like to have couple additional steps in Setup section of README.md. Steps I'd suggest are:

1. Short explanation of how to setup Google Maps API key of iOS app. I found this comment very helpful: flutter/flutter#17894 (comment). I know that there issue open for flutter repo to fix that, but it's open since May 2018, and not sure if it would be fixed soon :)
2. Short note regarding requirement to have io.flutter.embedded_views_preview enabled as explained here: flutter/flutter#26893 (comment). BUT this issue probably will be fixed soon, so we can have a note about that as well.

ps. I can pickup this issue if you guys think that this proposal makes sense.

[felangel]

@ezmeev I agree with the proposal 👍

If you have some free time it would be great if you could pick it up 💯

[jorgecoca]

Go for it! ;)