Add Slack badge

@jhump and @emcfarlane added one of these to the Vanguard README - it's
a good idea!

README.md

@@ -4,6 +4,7 @@ Connect
 [![Build](https://github.com/connectrpc/connect-go/actions/workflows/ci.yaml/badge.svg?branch=main)](https://github.com/connectrpc/connect-go/actions/workflows/ci.yaml)
 [![Report Card](https://goreportcard.com/badge/connectrpc.com/connect)](https://goreportcard.com/report/connectrpc.com/connect)
 [![GoDoc](https://pkg.go.dev/badge/connectrpc.com/connect.svg)](https://pkg.go.dev/connectrpc.com/connect)
+[![Slack](https://img.shields.io/badge/slack-buf-%23e01563)][slack]
 
 Connect is a slim library for building browser and gRPC-compatible HTTP APIs.
 You write a short [Protocol Buffer][protobuf] schema and implement your
@@ -181,3 +182,4 @@ Offered under the [Apache 2 license][license].
 [license]: https://github.com/connectrpc/connect-go/blob/main/LICENSE
 [protobuf]: https://developers.google.com/protocol-buffers
 [protocol]: https://connectrpc.com/docs/protocol
+[slack]: https://buf.build/links/slack