Add docs on edge case that breaks font loading iOS + expo

[WookieFPV]

I had an Issue after migrating from an ancient Version.
On iOS the Icons were not shown (on dev Builds).
After a lot of debugging I found the issue.
In the App entry point (index.js) the main App Component was imported before expo.
The order of imports broke font loading.

import App from './App'; //<-- importing this before expo broke the Icon loading
import { registerRootComponent } from 'expo'; 

registerRootComponent(App);

This is probably a rare edge cases but not easy to debug & solve.

Another workaround was to call the rnvi-update-plist because then the fonts were already included at compile time and dynamic loading was needed.

==> This PR adds a warning for this (probably rare) Edge cases.

[johnf]

@vonovak I'm happy with this PR but wanted to check if there was a way that we could avoid the issue or maybe warn about it at runtime?