add note that read class uses mixin

doc/source/contributing/icepyx_internals.rst

@@ -5,10 +5,11 @@ Authentication
 --------------
 Authentication in icepyx is handled using a Mixin class. A Mixin class is a class
 which defines functionality that may be desired by multiple other classes within 
-a library. For example, at this time both the Query and Variables classes need
+a library.
+For example, at this time the Query, Variables, and Read classes need
 to be able to authenticate. Instead of defining the same properties and
 functionality twice, icepyx has an EarthdataAuthMixin class that is inherited
-by both modules.
+by any modules that need an Earthdata login.
 
 **Property Access**