[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: Need to add a point below "Watch", by copy-paste below line:

- Discuss in the [Community Forum](https://discuss.layer5.io).

Give preview as below:

• Discuss in the Community Forum.

Environment:

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

[leecalcote]

Lgtm