Mention PostgreSQL backend in docs Search index

Updates the Search docs index to mention the new PostgreSQL backend

CONTRIBUTORS.rst

@@ -239,6 +239,7 @@ Contributors
 * Olly Willans
 * Ben Enright
 * Alice Rose
+* John Franey
 
 Translators
 ===========

docs/topics/search/index.rst

@@ -38,6 +38,6 @@ See :ref:`wagtailsearch_searching`.
 Backends
 ========
 
-Wagtail provides two backends for storing the search index and performing search queries: Elasticsearch and the database. It's also possible to roll your own search backend.
+Wagtail provides three backends for storing the search index and performing search queries: Elasticsearch, the database, and PostgreSQL (Django >=1.10 required). It's also possible to roll your own search backend.
 
 See :ref:`wagtailsearch_backends`