Doc update: Bumping usdz ver to 1.3 with AVIF support

(Internal change: 2345635)

docs/spec_usdz.rst

@@ -5,7 +5,7 @@ Usdz File Format Specification
 .. include:: rolesAndUtils.rst
 .. include:: <isonum.txt>
 
-Copyright |copy| 2018, Pixar Animation Studios,  *version 1.2*
+Copyright |copy| 2018, Pixar Animation Studios,  *version 1.3*
 
 .. contents:: :local:
 
@@ -198,6 +198,7 @@ Clients wishing to deliver "streamable content" `may wish to consider other layo
         * Otherwise, the packaging API will fail to run unless the Default
           Layer is manually specified.
 
+.. _spec_usdz_file_types:
 
 File Types
 ----------
@@ -408,3 +409,13 @@ constraints. The toolset also includes:
       enforce stricter "web-compliant" rules that disallow certain advanced USD
       features that web-browsers do not yet support.
 
+Changes, by Version
+===================
+
+Version 1.3 - Current Head
+--------------------------
+
+From version 1.2:
+
+    * :ref:`Adds AV1 Image (AVIF) file support <spec_usdz_file_types>`. 
+      AVIF is now a supported file type for images/textures.