Update instructions on how to join the Slack channel (apache#2219)

* add instructions

Signed-off-by: remzi <13716567376yh@gmail.com>

* fmt

Signed-off-by: remzi <13716567376yh@gmail.com>

* update discord link

Signed-off-by: remzi <13716567376yh@gmail.com>

README.md

@@ -51,7 +51,11 @@ You can find more details about each crate in their respective READMEs.
 The `dev@arrow.apache.org` mailing list serves as the core communication channel for the Arrow community. Instructions for signing up and links to the archives can be found at the [Arrow Community](https://arrow.apache.org/community/) page. All major announcements and communications happen there.
 
 The Rust Arrow community also uses the official [ASF Slack](https://s.apache.org/slack-invite) for informal discussions and coordination. This is
-a great place to meet other contributors and get guidance on where to contribute. Join us in the `#arrow-rust` channel.
+a great place to meet other contributors and get guidance on where to contribute. Join us in the `#arrow-rust` channel and feel free to ask for an invite via:
+
+1. the `dev@arrow.apache.org` mailing list
+2. the [GitHub Discussions][discussions]
+3. the [Discord channel](https://discord.gg/YAb2TdazKQ)
 
 Unlike other parts of the Arrow ecosystem, the Rust implementation uses [GitHub issues][issues] as the system of record for new features
 and bug fixes and this plays a critical role in the release process.
@@ -68,3 +72,4 @@ There is more information in the [contributing] guide.
 [datafusion-readme]: https://github.com/apache/arrow-datafusion/blob/master/README.md
 [ballista-readme]: https://github.com/apache/arrow-ballista/blob/master/README.md
 [issues]: https://github.com/apache/arrow-rs/issues
+[discussions]: https://github.com/apache/arrow-rs/discussions