[FLINK-11972] [docs] Add necessary notes about running streaming buck…

…eting end-to-end test in README

This closes apache#8027.

flink-end-to-end-tests/README.md

@@ -33,6 +33,12 @@ $ FLINK_DIR=<flink dir> flink-end-to-end-tests/run-single-test.sh your_test.sh a
 
 **NOTICE**: Please _DON'T_ run the scripts with explicit command like ```sh run-nightly-tests.sh``` since ```#!/usr/bin/env bash``` is specified as the header of the scripts to assure flexibility on different systems.
 
+### Streaming bucketing test
+
+Before running this nightly test case (test_streaming_bucketing.sh), please make sure to run `mvn -DskipTests install` in the `flink-end-to-end-tests` directory, so jar files necessary for the test like `BucketingSinkTestProgram.jar` could be generated.
+
+What's more, starting from 1.8.0 release it's required to make sure that `HADOOP_CLASSPATH` is [correctly set](https://ci.apache.org/projects/flink/flink-docs-stable/ops/deployment/hadoop.html) or the pre-bundled hadoop jar has been put into the `lib` folder of the `FLINK_DIR` (You can find the binaries from the [Downloads page](http:https://flink.apache.org/downloads.html) on the Flink project site).
+
 ### Kubernetes test
 
 Kubernetes test (test_kubernetes_embedded_job.sh) assumes a running minikube cluster.