Docs update for Elasticsearch 6 support

docs/topics/search/backends.rst

@@ -97,12 +97,18 @@ Elasticsearch Backend
 
     Support for Elasticsearch 5.x was added
 
-Elasticsearch versions 2 and 5 are supported. Use the appropriate backend for your version:
+.. versionchanged:: TODO ADD VERSION HERE
+
+    Support for Elasticsearch 6.x was added
+
+Elasticsearch versions 2, 5 and 6 are supported. Use the appropriate backend for your version:
 
 ``wagtail.search.backends.elasticsearch2`` (Elasticsearch 2.x)
 
 ``wagtail.search.backends.elasticsearch5`` (Elasticsearch 5.x)
 
+``wagtail.search.backends.elasticsearch6`` (Elasticsearch 6.x)
+
 Prerequisites are the `Elasticsearch`_ service itself and, via pip, the `elasticsearch-py`_ package. The major version of the package must match the installed version of Elasticsearch:
 
 .. _Elasticsearch: https://www.elastic.co/downloads/elasticsearch
@@ -115,6 +121,10 @@ Prerequisites are the `Elasticsearch`_ service itself and, via pip, the `elastic
 
   pip install "elasticsearch>=5.0.0,<6.0.0"  # for Elasticsearch 5.x
 
+.. code-block:: sh
+
+  pip install "elasticsearch>=6.0.0,<7.0.0"  # for Elasticsearch 6.x
+
 The backend is configured in settings:
 
 .. code-block:: python