README.md: Add sourcegraph badge

Author: kisielk

README.md

@@ -1,6 +1,8 @@
 securecookie
 ============
 [![GoDoc](https://godoc.org/github.com/gorilla/securecookie?status.svg)](https://godoc.org/github.com/gorilla/securecookie) [![Build Status](https://travis-ci.org/gorilla/securecookie.png?branch=master)](https://travis-ci.org/gorilla/securecookie)
+[![Sourcegraph](https://sourcegraph.com/github.com/gorilla/securecookie/-/badge.svg)](https://sourcegraph.com/github.com/gorilla/securecookie?badge)
+
 
 securecookie encodes and decodes authenticated and optionally encrypted 
 cookie values.