Add a note about creating new tests

Author: virtuald

README.md

@@ -88,6 +88,8 @@ If you find an bug, we encourage you to submit a pull request! New
 changes will only be accepted if there are tests to cover the change you
 made (and if they don’t break existing tests).
 
+It's really easy to add new tests, see the [README in the tests directory](tests/README.md).
+
 Author
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