Provide `iconSet` props supported values in Readme

[skychavda]

@djchie I am using your package and it's working fine but I have an issue when I try to use FontAwesome 5 from react-native-vector-image.

<StarRating
    disabled={false}
    halfStarEnabled
    maxStars={5}
    halfStar={'star-half-alt'}
    iconSet='FontAwesome5'
    rating={starCount}
    selectedStar={(rating) => setStarCount(rating)}
    fullStarColor={colors.yellow}
    halfStarColor={colors.yellow}
    emptyStarColor={colors.yellow}
/>

I wrote like this in iconSet but it gave an error while rendering saying that Element type is invalid. So can you provide the supporting prop value for the same in Read.me

It will be helped.

Thank you.