Add note to README about testing

README.md

@@ -47,3 +47,15 @@ cryptographically strong. Use this package to interoperate with systems that
 already uses HMAC SHA-1 and not for implementing new systems.
 
 There are stronger cryptographic algorithms like HMAC SHA-2, and [this](https://github.com/ktonon/elm-crypto) Elm package implements it.
+
+## Testing
+
+This package uses doc tests, which can be tested using [elm-verify-examples].
+To run all tests (assuming `elm-test` and `elm-verify-examples` are installed):
+
+```bash
+cd elm-hmac-sha1
+elm-verify-examples --fail-on-warn && elm-test
+```
+
+[elm-verify-examples]: https://github.com/stoeffel/elm-verify-examples