README.md: Shift Index discussion to the external-backend section

We use Index for third-party search engines.  For the 'sqlalchemy'
special case, we use SQLAlchemyIndex.

Author: wking

README.md

@@ -183,6 +183,9 @@ common:
   search_backend: foo.registry.index.xapian
 ```
 
+In this case, the module is imported, and an instance of its `Index`
+class is used as the search backend.
+
 #### sqlalchemy
 
 Use [SQLAlchemy][] as the search backend.
@@ -198,9 +201,6 @@ common:
   sqlalchemy_index_database: sqlite:////tmp/docker-registry.db
 ```
 
-In this case, the module is imported, and an instance of its `Index`
-class is used as the search backend.
-
 ### Mirroring Options
 
 All mirror options are placed in a `mirroring` section.