chore: add coveralls badge to readme

Author: cameri

README.md

@@ -1,5 +1,9 @@
 # [nostr-ts-relay](https://github.com/Cameri/nostr-ts-relay)
 
+<p align="center">
+<a href='https://coveralls.io/github/Cameri/nostr-ts-relay?branch=main'><img src='https://coveralls.io/repos/github/Cameri/nostr-ts-relay/badge.svg?branch=main' alt='Coverage Status' /></a>
+</p>
+
 This is a [nostr](https://github.com/fiatjaf/nostr) relay, written in
 Typescript.