Add to readme explanation about common error message
#### Attempt to insert nil object from objects[0]' was thrown while invoking getImageForFont on target RNVectorIcons - IOS
- If you see this error message:
> Exception '*** -[__NSPlaceholderDictionary initWithObjects:forKeys:count:]: attempt to insert nil object from objects[0]' was thrown while invoking getImageForFont on target RNVectorIcons with params (
> FontAwesome,
> "\Uf0c9",
> 23,
> 4294967295,
> 102,
> 103
> )
>
Make sure the Fonts are added to Xcode properly ([as explained here](https://github.com/oblador/react-native-vector-icons#ios-setup)).
** You can know which font is missing by the name of the first params (In the example above - FontAwesome)
**
Add to readme explanation about common error message