add read-me help for the ones that has install the flink-ml pip dependencies

[queyuexzy]

What is the purpose of the change

(For example: This pull request makes user can stop workflow externally.)

Brief change log

(for example:)

• Send StopDagEvent to Airflow scheduler via notification service
• Once received StopDagEvent, Airflow scheduler kills all running dag runs

Verifying this change

(Please pick either of the following options)

This change is a trivial rework / code cleanup without any test coverage.

(or)

This change is already covered by existing tests, such as (please describe tests).

(or)

This change added tests.