Clarify that one needs to call gerrit_api in the README.md

Change-Id: Ib209401e37ae4ce49670ab7886aeb8491012d99d
diff --git a/README.md b/README.md
index 7514661..d9d927f 100644
--- a/README.md
+++ b/README.md
@@ -28,6 +28,7 @@
 )
 load("@com_github_davido_bazlets//:gerrit_api.bzl",
      "gerrit_api")
+gerrit_api()
 ```
 
 Another option is to consume snapshot version of gerrit plugin API from local