Add to README new error explanation

[eidan66]

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)
**