Make first example of running e2e tests include longer timeout ⏱️

I foolishly tried to use the example at the top of the test README when
I was trying to run the timeout tests (for the tektoncd#731 investigation) and I
was sad when it failed b/c the default timeout was too short. This is
the timeout we are using when running CI anyway!

test/README.md

@@ -10,7 +10,7 @@ ko apply -f ./config/
 go test ./...
 
 # Integration tests (against your current kube cluster)
-go test -v -count=1 -tags=e2e ./test
+go test -v -count=1 -tags=e2e -timeout=20m ./test
 ```
 
 ## Unit tests