Update changelog for 1.6.0 release (#498)

Cutting a release so we can reduce the noise in stacktraces.

CHANGELOG.md

@@ -1,5 +1,13 @@
 # Changelog
 
+## v1.6.0 (30 Aug 2017)
+
+Enhancements:
+
+* [#491][]: Omit zap stack frames from stacktraces.
+* [#490][]: Add a `ContextMap` method to observer logs for simpler
+  field validation in tests.
+
 ## v1.5.0 (22 Jul 2017)
 
 Enhancements:
@@ -243,3 +251,5 @@ upgrade to the upcoming stable release.
 [#465]: https://github.com/uber-go/zap/pull/465
 [#460]: https://github.com/uber-go/zap/pull/460
 [#470]: https://github.com/uber-go/zap/pull/470
+[#490]: https://github.com/uber-go/zap/pull/490
+[#491]: https://github.com/uber-go/zap/pull/491