docs: slack invite link on Readme

Let users join slack from the link provided in the Readme

the link of the invite is already in the template when we create an issue, but users that have failed a first time to connect to slack won't always think that the invite link works

Author: fredericrous

README.md

@@ -15,7 +15,7 @@ If you find this lib useful, please donate [PayPal](https://www.paypal.me/alaind
 [![Donate](https://img.shields.io/badge/Donate-PayPal-green.svg)](https://www.paypal.me/alaind831)
 [![Donate](https://img.shields.io/badge/Donate-Venmo-g.svg)](https://www.venmo.com/adumesny)
 
-Join us on Slack: https://gridstackjs.slack.com
+Join us on Slack: [https://gridstackjs.slack.com](https://join.slack.com/t/gridstackjs/shared_invite/zt-1jobnclsy-FqGGeLX5dFPM_ZQzTunBsw)
 
 <!-- [![Slack Status](https://gridstackjs.com/badge.svg)](https://gridstackjs.slack.com) -->
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