[README] Need to add `Community Forum`

[aminoxix]

Description

README.md is the first file one should read when starting a new project. Our community launched Discussion forum for help seekers; needs to publish it officially on README.md for convenience.

Screenshots

Current behaviour:

Desired behaviour:

Environment:

• OS: any [e.g. Ubuntu]
• Browser: any [e.g. Chrome, Safari]
• Version: any [e.g. 22]
• Device: any [e.g. laptop, iPhone 8]

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Reference:

• Take reference to this pull request