Add guidance on using and creating relation types

Using URIs for custom relation types is often a little
intimidating, and frequently ignored.  Provide some guidance
on some simple approaches to make it easier for hyper-schema
authors to use this now-required field correctly.

Author: handrews

jsonschema-hyperschema.xml

@@ -3,6 +3,7 @@
 <!ENTITY rfc2046 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.2046.xml">
 <!ENTITY rfc2119 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.2119.xml">
 <!ENTITY rfc3986 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.3986.xml">
+<!ENTITY rfc4151 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.4151.xml">
 <!--<!ENTITY rfc4287 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.4287.xml">-->
 <!--<!ENTITY rfc5226 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.5226.xml">-->
 <!ENTITY rfc5789 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.5789.xml">
@@ -669,13 +670,24 @@
 
                 <t>
                     Relationship definitions are not normally media type
-                    dependent, and users are encouraged to utilize existing
-                    accepted relation definitions.
+                    dependent, and users are encouraged to utilize the most
+                    suitable existing accepted relation definitions.
                 </t>
 
+                <t>
+                    When no registered relation (aside from "related") applies, users are
+                    encouraged to mint their own extension relation types, as described in
+                    <xref target="RFC5988">section 4.2 of RFC 5988</xref>.  The simplest
+                    approaches for choosing link relation type URIs are to either use
+                    a URI scheme that is already in use to identify the system's primary
+                    resources, or to use a human-readable, non-dereferenceable URI scheme
+                    such as <xref target="RFC4151">"tag", defined by RFC 4151</xref>.
+                    Extension relation type URIs need not be dereferenceable, even when
+                    using a scheme that allows it.
+                </t>
                 <figure>
                     <preamble>
-                        For example, if a hyper-schema is defined:
+                        As an example of registered relation types, if a hyper-schema is defined:
                     </preamble>
                     <artwork>
 <![CDATA[{
@@ -1216,6 +1228,7 @@ GET /foo/
         <references title="Informative References">
             &rfc2046;
             <!--&rfc5226;-->
+            &rfc4151;
             &rfc5789;
             &rfc5988;
             &rfc7231;