Document enabling debug output (#391)

Document enabling debug output

Author: WillGibson

README.rst

@@ -149,6 +149,20 @@ You can exclude files from mutation in `setup.cfg`:
         *__tests.py
 
 
+Enable debug output (increase verbosity)
+----------------------------------------
+
+By default, mutmut "swallows" all the test output etc. so that you get a nice clean output.
+
+If you want to see all the detail to aid with debugging, you can set `debug` to `true` in your configuration.
+Note that not all displayed errors are necessarily bad. In particular test runs of the mutated code will lead
+to failing tests.
+
+.. code-block::
+
+    debug=true
+
+
 Whitelisting
 ------------