Add documentation explaining set_default behavior

Author: zerowidth

README.md

@@ -84,6 +84,8 @@ class MyExperiment
 end
 ```
 
+When `Scientist::Experiment` is included in a class, it automatically sets it as the default implementation via `Scientist::Experiment.set_default`. This `set_default` call is is skipped if you include `Scientist::Experiment` in a module.
+
 Now calls to the `science` helper will load instances of `MyExperiment`.
 
 ### Controlling comparison