[jnigen] Add Coverage badge (https://github.com/dart-lang/jnigen/issues/205)

pkgs/jnigen/README.md

diff --git a/pkgs/jnigen/README.md b/pkgs/jnigen/README.md
index d599e7c..94a0855 100644
--- a/pkgs/jnigen/README.md
+++ b/pkgs/jnigen/README.md
@@ -1,4 +1,5 @@
 [![Build Status](https://github.com/dart-lang/jnigen/workflows/Dart%20CI/badge.svg)](https://github.com/dart-lang/jnigen/actions?query=workflow%3A%22Dart+CI%22+branch%3Amain)
+[![Coverage Status](https://coveralls.io/repos/github/dart-lang/jnigen/badge.svg?branch=main)](https://coveralls.io/github/dart-lang/jnigen?branch=main)
 
 ## Introduction
 Experimental bindings generator for Java bindings through dart:ffi and JNI.