Update README.md (istio#2564)

Automatic merge from submit-queue.

Update README.md

Documenting istio/test-infra#662

tests/e2e/README.md

@@ -2,6 +2,20 @@
 
 This directory contains Istio end-to-end tests and associated test framework.
 
+Only an E2E smoke test (bookinfo test with auth enabled) is automatically triggered in Before-Merge stage for every PR. The full suite of E2E tests are run in After-Merge stage only. Their results can be found in https://prow.istio.io/ and https://k8s-testgrid.appspot.com/istio
+
+# Running E2E tests in PR stage
+
+E2E-suites can be manually triggered in PR stage with specific command:
+
+`/test e2e-suite-rbac-no_auth`
+
+`/test e2e-suite-rbac-auth`
+
+`/test e2e-cluster_wide-auth`
+
+> NOTE: Please do "/test e2e-*" only after "prow/istio-presubmit" completes, as the e2e tests need the artifacts generated by istio-presubmit.
+
 # Running E2E tests on your own kubernets cluster
 
 NOTE: the e2e tests might not run on a Mac because istioctl-osx, needed for test execution, is only built for release