Add gorilla changelog entry
## ChangeLog
+* Added support for [gorilla/mux](http://github.com/gorilla/mux) in the new `nrgorilla`
+ package.
+ * [Documentation](http://godoc.org/github.com/newrelic/go-agent/_integrations/nrgorilla/v1)
+ * [Example](examples/_gorilla/main.go)
+
## 1.6.0
* Added support for custom error messages and stack traces. Errors provided
