Update usage to reflect the recent addition of the pytest plugin (#128)

Author: tomhamiltonstubber

docs/usage.md

@@ -113,14 +113,17 @@ To manually add `debug` to `__builtins__`, add the following to `sitecustomize.p
 which is always imported.
 
 ```py
-# add devtools `debug` function to builtins
-import builtins
-try:
-    from devtools import debug
-except ImportError:
-    pass
-else:
-    setattr(builtins, 'debug', debug)
+import sys
+# we don't install here for pytest as it breaks pytest, it is
+# installed later by a pytest fixture
+if not sys.argv[0].endswith('pytest'):
+    import builtins
+    try:
+        from devtools import debug
+    except ImportError:
+        pass
+    else:
+        setattr(builtins, 'debug', debug)
 ```
 
 The `ImportError` exception is important since you'll want python to run fine even if *devtools* isn't installed.