Add Zulip to README (#52403)

As
[suggested](JuliaLang/www.julialang.org#1982 (comment))
by @jariji

Placing it above Slack because

- Zulip does not eat messages (the slackhole is a drain of community
resources, esp. on the #helpdesk channel)
- Zulip is FOSS
- Zulip is a wee bit more feature complete (this is debatable, but I
hold it to be true)

Author: LilithHafner

README.md

@@ -48,6 +48,7 @@ and installing Julia, below.
 - **Documentation:** <https://docs.julialang.org>
 - **Packages:** <https://julialang.org/packages/>
 - **Discussion forum:** <https://discourse.julialang.org>
+- **Zulip:** <https://julialang.zulipchat.com/>
 - **Slack:** <https://julialang.slack.com> (get an invite from <https://julialang.org/slack/>)
 - **YouTube:** <https://www.youtube.com/user/JuliaLanguage>
 - **Code coverage:** <https://coveralls.io/r/JuliaLang/julia>