readme: update badges (#105)

Replace the Slack badge with a link to https://discuss.pomerium.com.
Update the broken build status and coverage badges.

README.md

@@ -1,6 +1,6 @@
 # Webauthn
 
-[![Go Report Card](https://goreportcard.com/badge/github.com/pomerium/webauthn)](https://goreportcard.com/report/github.com/pomerium/webauthn) [![GoDoc](https://pkg.go.dev/badge/github.com/pomerium/webauthn?status.svg)](https://pkg.go.dev/github.com/pomerium/webauthn?tab=doc) [![Coverage Status](https://coveralls.io/repos/github/pomerium/webauthn/badge.svg)](https://coveralls.io/github/pomerium/webauthn) [![pomerium chat](https://img.shields.io/badge/chat-on%20slack-blue.svg?style=flat&logo=slack)](http://slack.pomerium.io) [![GitHub Actions](https://img.shields.io/github/workflow/status/pomerium/webauthn/Test?style=flat)](https://github.com/pomerium/webauthn/actions?query=workflow%3ATest) [![LICENSE](https://img.shields.io/github/license/pomerium/webauthn.svg)](https://github.com/pomerium/webauthn/blob/master/LICENSE)
+[![Go Report Card](https://goreportcard.com/badge/github.com/pomerium/webauthn)](https://goreportcard.com/report/github.com/pomerium/webauthn) [![GoDoc](https://pkg.go.dev/badge/github.com/pomerium/webauthn?status.svg)](https://pkg.go.dev/github.com/pomerium/webauthn?tab=doc) [![Coverage Status](https://coveralls.io/repos/github/pomerium/webauthn/badge.svg?branch=main)](https://coveralls.io/github/pomerium/webauthn?branch=main) [![discuss](https://img.shields.io/discourse/posts?server=https%3A%2F%2Fdiscuss.pomerium.com%2F&label=discuss)](https://discuss.pomerium.com/) [![GitHub Actions](https://img.shields.io/github/actions/workflow/status/pomerium/webauthn/test.yml?style=flat)](https://github.com/pomerium/webauthn/actions/workflows/test.yml) [![LICENSE](https://img.shields.io/github/license/pomerium/webauthn.svg)](https://github.com/pomerium/webauthn/blob/master/LICENSE)
 
 This [Go library](https://pkg.go.dev/github.com/pomerium/webauthn) implements [WebAuthn](https://www.w3.org/TR/webauthn/)/[FIDO2](https://fidoalliance.org/fido2/) spec as a [relying party](https://www.w3.org/TR/webauthn/#relying-party), and includes: