docs: add Slack community invite link to README

## Summary

### Why?

There is no discoverable entry point for people who want to ask questions, discuss design, or get help getting started with SubmitQueue. A public Slack workspace exists but is not linked anywhere in the repo, so newcomers land on the README with no way to reach the maintainers short of filing an issue.

### What?

Adds the shared Slack invite link to `README.md` in two places: a Slack badge alongside the existing CI / Go version / License badges, and a new `## Community` section between Project Status and Contributing that spells out what the channel is for.

Docs only — no code, build, or generated files are touched.

## Test Plan

Docs-only change; verified by reading the diff. The badge and section render will be visible in the PR preview on GitHub.

Author: behinddwalls

README.md

@@ -3,6 +3,7 @@
 [![CI](https://github.com/uber/submitqueue/actions/workflows/ci.yml/badge.svg)](https://github.com/uber/submitqueue/actions/workflows/ci.yml)
 [![Go Version](https://img.shields.io/github/go-mod/go-version/uber/submitqueue)](go.mod)
 [![License](https://img.shields.io/badge/License-Apache_2.0-blue.svg)](LICENSE)
+[![Slack](https://img.shields.io/badge/Slack-join%20the%20community-4A154B?logo=slack&logoColor=white)](https://join.slack.com/t/submitqueue/shared_invite/zt-46gkqj682-7zcQphxm2pYqkjDo9lbmYA)
 
 SubmitQueue is a high-performance speculative merge queue that keeps your trunk consistently green at scale. Rather than validating changes one at a time, SubmitQueue speculatively rebases and validates multiple changes in parallel against predicted future states of HEAD. When validations pass, changes land automatically. When they fail, SubmitQueue isolates the offending change and retries the rest — all without human intervention.
 
@@ -50,6 +51,10 @@ See [service/README.md](service/README.md) for more examples including running i
 
 SubmitQueue is under active development. We welcome contributions and feedback.
 
+## Community
+
+Join us on Slack: [submitqueue.slack.com](https://join.slack.com/t/submitqueue/shared_invite/zt-46gkqj682-7zcQphxm2pYqkjDo9lbmYA) — questions, design discussions, and help getting started.
+
 ## Contributing
 
 See [CONTRIBUTING.md](CONTRIBUTING.md) for how to get started.