Release v1.6.0 (#324)

* Update version and changelog for v1.6.0
* Update docs
diff --git a/CHANGELOG b/CHANGELOG
index c0a6693..3a754d1 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -1,3 +1,24 @@
+v1.6.0
+  Version 1.6.0
+
+  Release to drop support for Python 2.6 and add support for google-auth.
+
+  - Support for Python 2.6 has been dropped. (#319)
+  - The credentials argument to discovery.build and discovery.build_from_document
+    can be either oauth2client credentials or google-auth credentials. (#319)
+  - discovery.build and discovery.build_from_document now unambiguously use the
+    http argument to make all requests, including the request for the discovery
+    document. (#319)
+  - The http and credentials arguments to discovery.build and
+    discovery.build_from_document are now mutually exclusive, eliminating a
+    buggy edge case. (#319)
+  - If neither http or credentials is specified to discovery.build and
+    discovery.build_from_document, then Application Default Credentials will
+    be used. The library prefers google-auth for this if it is available, but
+    can also use oauth2client's implementation. (#319)
+  - Fixed resumable upload failure when receiving a 308 response. (#312)
+  - Clarified the support versions of Python 3. (#316)
+
 v1.5.5
   Version 1.5.5