Add documentation for custom scalar types

[sirtoobii]

While searching for a way to have have generic a "Json Object" field, I found out that there is the concept of custom scalars and at first it seemed, as if this graphQL implementation does not support them. However, then I stumbled over #321 which eventually lead me to the scalars configuration property which does exactly that.

Having a custom scalar type for larger than 32bit integres could also help to properly resolve this issue: silverstripe/silverstripe-asset-admin#1299