README: Add panic example when using wrong dependency

README.md

@@ -6,7 +6,7 @@ and send the result down the request context (`context.Context`).
 
 **NOTE:** `jwtauth` requires [v2.7.0](https://github.com/dgrijalva/jwt-go/tree/v2.7.0) of the
 `github.com/dgrijalva/jwt-go` dependency. Please make sure to vendor the correct version
-into your project. There is a PR https://github.com/goware/jwtauth/pull/8 to support v3.0.0,
+into your project. If you see `panic: interface conversion: interface {} is *jwt.Token, not *jwt.Token` error, you don't have the required version. There is a PR https://github.com/goware/jwtauth/pull/8 to support v3.0.0,
 and it's quite close, but it needs another review after recent updates to jwt-go.
 
 This package uses the new `context` package in Go 1.7 also used by `net/http`