Add guidance on bundling and references

This should make clear what use cases are considered valid
and safe, vs when someone is on their own in terms of figuring
that out.

Author: handrews

jsonschema-core.xml

@@ -3435,6 +3435,57 @@ User-Agent: product-name/5.4.1 so-cool-json-schema/1.0.2 curl/7.43.0
             </t>
         </section>
 
+        <section title="Manipulating schema documents and references">
+            <t>
+                Various tools have been created to rearrange schema documents
+                based on how and where references ("$ref") appear.  This appendix discusses
+                which use cases and actions are compliant with this specification.
+            </t>
+            <section title="Bundling schema resources into a single document">
+                <t>
+                    A set of schema resources intended for use together can be organized
+                    with each in its own schema document, all in the same schema document,
+                    or any granularity of document grouping in between.
+                </t>
+                <t>
+                    Numerous tools exist to perform various sorts of reference removal.
+                    A common case of this is producing a single file where all references
+                    can be resolved within that file.  This is typically done to simplify
+                    distribution, or to simplify coding so that various invocations
+                    of JSON Schema libraries do not have to keep track of and load
+                    a large number of resources.
+                </t>
+                <t>
+                    This transformation can be safely and reversibly done as long as
+                    all static references (e.g. "$ref") use URI-references that resolve
+                    to canonical URIs, and all schema resources have an absolute-URI
+                    as the "$id" in their root schema.
+                </t>
+                <t>
+                    With these conditions met, each external resource can be copied
+                    under "$defs", without breaking any references among the resources'
+                    schema objects, and without changing any aspect of validation or
+                    annotation results.  The names of the schemas under "$defs" do
+                    not affect behavior, assuming they are each unique, as they
+                    do not appear in canonical URIs for the embedded resources.
+                </t>
+            </section>
+            <section title="Reference removal is not always safe">
+                <t>
+                    Attempting to remove all references and produce a single schema document does not,
+                    in all cases, produce a schema with identical behavior to the original form.
+                </t>
+                <t>
+                    Since "$ref" is now treated like any other keyword, with other keywords allowed
+                    in the same schema objects, fully supporting non-recursive "$ref" removal in
+                    all cases can require relatively complex schema manipulations.  It is beyond
+                    the scope of this specification to determine or provide a set of safe "$ref"
+                    removal transformations, as they depend not only on the schema structure
+                    but also on the intende usage.
+                </t>
+            </section>
+        </section>
+
         <section title="Working with vocabularies">
             <section title="Best practices for vocabulary and meta-schema authors">
                 <t>