Should provide safer examples for custom type converters

[kcoppock]

On the documentation for custom types:

https://github.com/bluelinelabs/LoganSquare/blob/master/docs/TypeConverters.md

It mentions using TestEnum.values()[index] or TestEnum.valueOf(string) to do the conversion. However, these can both cause uncaught exceptions (ArrayIndexOutOfBounds for the index lookup, IllegalArgumentException for the String lookup) if the values on the remote service ever change; for example, if a new value is added, but an older client doesn't have an enum value for it yet.