Update README.md

Explicitly mentions v4 in Usage

Author: rob-deutsch

README.md

@@ -21,7 +21,7 @@
 <br>
 
 ### Usage
-[Go Modules](https://blog.golang.org/using-go-modules) are mandatory for using Pion WebRTC. So make sure you set `export GO111MODULE=on`, and explicitly specify `/v2` or `/v3` when importing.
+[Go Modules](https://blog.golang.org/using-go-modules) are mandatory for using Pion WebRTC. So make sure you set `export GO111MODULE=on`, and explicitly specify `/v4` (or an earlier version) when importing.
 
 
 **[example applications](examples/README.md)** contains code samples of common things people build with Pion WebRTC.