add coverage explanation

examples/tutorial/README.rst

@@ -71,6 +71,11 @@ Test
     $ pytest
 
 Run with coverage report::
+    If you haven't install `coverage`_.
+
+.. _coverage: https://coverage.readthedocs.io
+
+    $ pip install coverage # skip this line if you already installed coverage.
 
     $ coverage run -m pytest
     $ coverage report