Update docs to require full namespace

FactoryBot throws an error when defining factories that refer to classes
within modules in non-Rails applications. FactoryBot will work, but the
class used by the factory must be explicitly stated in the full
namespace.

GETTING_STARTED.md

@@ -219,6 +219,26 @@ It is also possible to explicitly specify the class:
 factory :admin, class: "User"
 ```
 
+Explicit specification of the class, _with the full namespace_, is necessary when defining factories for classes within modules in non-Rails applications:
+
+```
+# foo/bar.rb
+module Foo
+  class Bar
+    ...
+  end
+end
+
+# factories.rb
+FactoryBot.define do
+   factory :bar, class: 'Foo::Bar' do
+    ...
+  end
+end
+```
+
+If the full namespace is not provided in the `factory` statement, you will receive a `NameError: uninitialized constant Bar` error.
+
 You can pass a constant as well, if the constant is available (note that this
 can cause test performance problems in large Rails applications, since
 referring to the constant will cause it to be eagerly loaded).