README: add a direct mention of the community calendar

We keep bringing it up on Slack and Twitter,
but it's surprisingly not yet directly mentioned in our main repo
nor is it mentioned on the cuelang.org website yet.

It should probably be mentioned in both.
For now, adding it to the README is easy enough,
and it fits well next to the Discussions and Slack
in terms of staying in touch with the community.

I decided against placing it in CONTRIBUTING.md
since the community calendar is for all users,
not just those opening issues or sending patches.

Signed-off-by: Daniel Martí <[email protected]>
Change-Id: Ie46bef92fed1fc422f9037a472e43799e15e57e8
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1171095
TryBot-Result: CUEcueckoo <[email protected]>
Reviewed-by: Paul Jolly <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>

README.md

@@ -153,8 +153,8 @@ Conduct](https://cuelang.org/docs/contribution_guidelines/conduct).
 You can get in touch with the cuelang community in the following ways:
 
 - Ask questions via [GitHub Discussions](https://github.com/cue-lang/cue/discussions)
-- Chat with us on our
-  [Slack workspace](https://join.slack.com/t/cuelang/shared_invite/enQtNzQwODc3NzYzNTA0LTAxNWQwZGU2YWFiOWFiOWQ4MjVjNGQ2ZTNlMmIxODc4MDVjMDg5YmIyOTMyMjQ2MTkzMTU5ZjA1OGE0OGE1NmE).
+- Chat with us on our [Slack workspace](https://join.slack.com/t/cuelang/shared_invite/enQtNzQwODc3NzYzNTA0LTAxNWQwZGU2YWFiOWFiOWQ4MjVjNGQ2ZTNlMmIxODc4MDVjMDg5YmIyOTMyMjQ2MTkzMTU5ZjA1OGE0OGE1NmE).
+- Subscribe to our [Community Calendar](https://cuelang.org/s/community-calendar) for community calls, demos, office hours, etc
 
 ---