Contributing (#65)

* [docs update] How to view test coverage.

Author: avishalom

CONTRIBUTING.md

@@ -137,3 +137,18 @@ go test firebase.google.com/go/...
 ```
 
 This will execute both unit and integration test suites.
+
+### Test Coverage
+
+Coverage can be measured per package by passing the `-cover` flag to the test invocation:
+
+```bash
+go test -cover firebase.google.com/go/auth
+```
+
+To view the detailed coverage reports (per package):
+
+```bash
+go test -cover -coverprofile=coverage.out firebase.google.com/go
+go tool cover -html=coverage.out
+```