Add README.md for elasticdl_client folder. (#2129)

* Add READMD.md for elasticdl_client folder.

* Update according to the comments.

* Resolve comments.

Author: brightcoder01

elasticdl_client/README.md

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+# ElasticDL Client
+
+ElasticDL client is a command-line tool. Users can use it to prepare
+Docker images for distributed ElasticDL jobs and submit train, evaluate
+or predict jobs to a Kubernetes cluster.
+
+For more detailed instructions, please refer to the
+[tutorial](../docs/tutorials/get_started.md).