ARM Notification Group for Rust Compiler

1st chunk of `src/notification-groups/arm.md`

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# ARM notification group

**Github Label:** [O-ARM] <br>
**Ping command:** `@rustbot ping arm`


This list will be used to ask for help both in diagnosing and testing
ARM-related issues as well as suggestions on how to resolve
interesting questions regarding our ARM support.

The group also has an associated Zulip stream ([`#t-compiler/arm`])
where people can go to pose questions and discuss ARM-specific
topics.

So, if you are interested in participating, please sign up for the
ARM group! To do so, open a PR against the [rust-lang/team]
repository. Just [follow this example][eg], but change the username to
your own!

Title: ARM Notification Group for Rust Compiler

Summary

This text describes the ARM notification group for the Rust compiler project. It explains the group's purpose (diagnosing, testing, and resolving ARM-related issues), the ping command to reach the group (@rustbot ping arm), and the associated Zulip stream (#t-compiler/arm). It also provides instructions for joining the group by submitting a pull request to the rust-lang/team repository.