README: Add discord channel

[ChenlingLu]

Add a new community channel - Discord

[sre-bot]

All committers have signed the CLA.

[ti-chi-bot[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: breezewish

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files: - ~~[OWNERS](https://github.com/pingcap/community/blob/master/OWNERS)~~ [breezewish] Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment

[ti-chi-bot[bot]]

[LGTM Timeline notifier]

Timeline:

• 2023-08-31 05:01:11.958251553 +0000 UTC m=+1989636.507267542: :ballot_box_with_check: agreed by breezewish.