Add a missing docstring

Lib/collections/__init__.py

+'''This module implements specialized container datatypes providing
+alternatives to Python’s general purpose built-in containers, dict,
+list, set, and tuple.
+
+* namedtuple   factory function for creating tuple subclasses with named fields
+* deque        list-like container with fast appends and pops on either end
+* ChainMap     dict-like class for creating a single view of multiple mappings
+* Counter      dict subclass for counting hashable objects
+* OrderedDict  dict subclass that remembers the order entries were added
+* defaultdict  dict subclass that calls a factory function to supply missing values
+* UserDict     wrapper around dictionary objects for easier dict subclassing
+* UserList     wrapper around list objects for easier list subclassing
+* UserString   wrapper around string objects for easier string subclassing
+
+'''
+
 __all__ = ['deque', 'defaultdict', 'namedtuple', 'UserDict', 'UserList',
             'UserString', 'Counter', 'OrderedDict', 'ChainMap']