Introduce the LDO in the introduction

and note that it is a serialization of the abstract link
model from RFC 5988bis

Author: handrews

jsonschema-hyperschema.xml

@@ -102,7 +102,15 @@
                 keywords.  The term "hyper-schema" on its own refers to a JSON Hyper-Schema
                 within the scope of this specification.
             </t>
-
+            <t>
+                The primary mechanism introduced for specifying links is the Link Description
+                Object (LDO), which is a serialization of the abstract link model
+                defined in <xref target="I-D.nottingham-rfc5988bis">RFC 5988bis, section 2</xref>.
+                <cref>
+                    This section references the work-in-progress RFC5988bis, with the expectation
+                    that it will reach RFC before this specification does.
+                </cref>
+            </t>
             <t>
                 This specification will use the concepts, syntax, and terminology defined by the
                 <xref target="json-schema">JSON Schema core</xref> and