Update README.md
diff --git a/README.md b/README.md
index 9bb0860..b3427ab 100644
--- a/README.md
+++ b/README.md
@@ -2,6 +2,7 @@
=========
[](https://travis-ci.org/google/benchmark)
[](https://ci.appveyor.com/project/google/benchmark/branch/master)
+[](https://coveralls.io/r/google/benchmark)
A library to support the benchmarking of functions, similar to unit-tests.