add some clarifications to the custom Scalar type doc

Author: defrex

docs/pages/docs/basic-types.md

@@ -39,6 +39,7 @@ If you want to create a DateTime Scalar Type just type:
 
 ```python
 import datetime
+from graphene.core.classtypes import Scalar
 from graphql.core.language import ast
 
 class DateTime(Scalar):
@@ -58,6 +59,16 @@ class DateTime(Scalar):
         return datetime.datetime.strptime(value, "%Y-%m-%dT%H:%M:%S.%f")
 ```
 
+Unlike the other basic types, custom `Scalar` types should not be instantiated
+when used.
+
+```python
+import graphene
+
+class Post(graphene.ObjectType):
+    created = graphene.Field(DateTime)
+```
+
 ## Mounting in ClassTypes
 
 This types if are mounted in a `ObjectType`, `Interface` or `Mutation`,