Merge pull request OAI#1062 from OAI/issue-1025

clarified `null`

Author: Ron

versions/3.0.md

@@ -139,7 +139,7 @@ By convention, it is RECOMMENDED that the OpenAPI Specification (OAS) file be na
 
 Primitive data types in the OAS are based on the types supported by the [JSON Schema Specification Wright Draft 00](https://tools.ietf.org/html/draft-wright-json-schema-00#section-4.2). 
 Note that `integer` as a type is also supported and is defined as a JSON number without a fraction or exponent part. 
-`null` is not supported as a value.
+`null` is not supported as a type (see [`nullable`](#schemaNullable) for an alternative solution).
 Models are described using the [Schema Object](#schemaObject) which is an extended subset of JSON Schema Specification Wright Draft 00.
 
 <a name="dataTypeFormat"></a>Primitives have an optional modifier property: `format`.