Add elasticsearch 5.x intrusion info in index.rst (#514)

for issue https://github.com/elastic/elasticsearch-py/issues/492

docs/index.rst

@@ -17,6 +17,9 @@ Compatibility
 The library is compatible with all Elasticsearch versions since ``0.90.x`` but you
 **have to use a matching major version**:
 
+For **Elasticsearch 5.0** and later, use the major version 5 (``5.x.y``) of the
+library.
+
 For **Elasticsearch 2.0** and later, use the major version 2 (``2.x.y``) of the
 library.
 
@@ -29,6 +32,9 @@ library.
 The recommended way to set your requirements in your `setup.py` or
 `requirements.txt` is::
 
+    # Elasticsearch 5.x
+    elasticsearch>=5.0.0,<6.0.0
+
     # Elasticsearch 2.x
     elasticsearch>=2.0.0,<3.0.0
 
@@ -38,7 +44,7 @@ The recommended way to set your requirements in your `setup.py` or
     # Elasticsearch 0.90.x
     elasticsearch<1.0.0
 
-The development is happening on ``master`` and ``1.x`` branches, respectively.
+The development is happening on ``master`` and ``2.x`` branches respectively.
 
 Example Usage
 -------------