Update readme.md

[Luwa-Tech]

In the "Community and Support" section, i replaced the line that mentions the community Slack with a note explaining that a @harness.io address is required to join the Slack community

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[bradrydzewski]

the Slack invitation link expired which is what caused the problem. the team generated a new link, which will allow joining Slack without a harness address. this change should be reflected here https://docs.gitness.com/support