Update README.md

Added to documentation stating that if you want to use group prefixes, then a definition without a group prefix must be created as well.

README.md

@@ -57,7 +57,7 @@ This is the usage guide for Factory Muffin 3.0. Within this guide, you will see
 
 You can define model factories using the `define` function. You may call it like this: `$fm->define('Fully\Qualifed\ModelName')->addDefinitions('foo', 'bar')`, where `foo` is the name of the attribute you want set on your model, and `bar` describes how you wish to generate the attribute. You may also define multiple attributes at once like this: `$fm->define('Fully\Qualifed\ModelName')->setDefinitions('foo', 'bar')`. Note that both functions append to the internal attributes definition array rather than replacing it. Please see the generators section for more information on how this works.
 
-You can also define multiple different model definitions for your models. You can do this by prefixing the model class name with your "group" followed by a colon. This results in you defining your model like this: `$fm->define('myGroup:Fully\Qualifed\ModelName')->addDefinitions('bar', 'baz')`. You don't have to entirely define your model here because we will first look for a definition without the group prefix, then apply your group definition on top of that definition, overriding attribute definitions where required.
+You can also define multiple different model definitions for your models. You can do this by prefixing the model class name with your "group" followed by a colon. This results in you defining your model like this: `$fm->define('myGroup:Fully\Qualifed\ModelName')->addDefinitions('bar', 'baz')`. You don't have to entirely define your model here because we will first look for a definition without the group prefix, then apply your group definition on top of that definition, overriding attribute definitions where required. Note that if you want to use group prefixes, you *must* also create a definition without the group prefix as well.
 
 We have provided a nifty way for you to do this in your tests. PHPUnit provides a `setupBeforeClass` function. Within that function you can call `$fm->loadFactories(__DIR__ . '/factories');`, and it will include all files in the factories folder. Within those php files, you can put your definitions (all your code that calls the define function). The `loadFactories` function will throw a `League\FactoryMuffin\Exceptions\DirectoryNotFoundException` exception if the directory you're loading is not found.