update README install instructions for v2 (#91)

diff --git a/README.md b/README.md
index c3bb2e1..aae170f 100644
--- a/README.md
+++ b/README.md
@@ -10,7 +10,7 @@
 To install the API extensions, use:
 
 ```
-go get -u github.com/googleapis/gax-go
+go get -u github.com/googleapis/gax-go/v2
 ```
 
 **Note:** Application code will rarely need to use this library directly,