added extensions

Author: fehguy

versions/3.0.md

@@ -1150,6 +1150,7 @@ href      |  url   | a relative or absolute URL to a linked resource. This field
  operationId| string | the name of an _existing_, resolvable OAS operation, as defined with a unique `operationId`.  This field is mutually exclusive with the `href` field.  Relative `href` values _may_ be used to locate an existing Operation Object in the OAS.
 parameters  | Link Parameters Object | an Object representing parameters to pass to an operation as specified with `operationId` or identified via `href`.
 description | string | a description of the link, supports GFM.
+^x-         | Any    | Allows extensions to the OpenAPI Schema. The field name MUST begin with `x-`, for example, `x-internal-id`. The value can be `null`, a primitive, an array or an object. See [Specification Extensions](#specificationExtensions) for further details.
 
 Locating a linked resource may be performed by either a `href` or `operationId`. In the case of an `operationId`, it must be unique and resolved in the scope of the OAS document.  Because of the potential for name clashes, consider the `href` syntax as the preferred method for specifications with external references.