Update README.md to tell users how to start an open discussion if they have some questions.
Update README.md to tell users how to start an open discussion if they have some questions.

Add content as below:
Please feel free to ask any questions or queries through "issues" label of "https://github.com/HuaweiBigData/astro" if you are interested in this project. And very welcome to participate in contribution on this project which is 100% open for everyone.
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