Add Slack invite link (#2704)

Author: yuyiming

README.rst

@@ -330,6 +330,7 @@ Getting involved
 ----------------
 
 - Read `development guide <https://docs.pymars.org/en/latest/development/index.html>`_.
+- Join our Slack workgroup: `Slack <https://join.slack.com/t/mars-computing/shared_invite/zt-13ldfh53l-mWBp4YhN7XaDpXjnKmpmeQ>`_.
 - Join the mailing list: send an email to `[email protected]`_.
 - Please report bugs by submitting a `GitHub issue`_.
 - Submit contributions using `pull requests`_.