Add README.md for package:collections_helpers.

pkg/collection_helpers/README.md

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+Helper libraries for working with collections.
+
+The `collection_helpers` package contains a number of separate libraries
+with utility functions and classes that makes working with collections easier.
+
+## Using
+
+The `collection_helpers` library can be imported as separate libraries, or
+in totality:
+
+    import 'package:collection_helpers/equality.dart';
+    import 'package:collection_helpers/algorithms.dart';
+    import 'package:collection_helpers/wrappers.dart';
+
+or
+
+    import 'package:collection_helpers/all.dart';
+
+## Equality
+
+The equality library gives a way to specify equality of elements and
+collections.
+
+Collections in Dart have no inherent equality. Two sets are not equal, even
+if they contain exactly the same objects as elements.
+
+The equality library provides a way to say define such an equality. In this
+case, for example, `const SetEquality(const IdentityEquality())` is an equality
+that considers two sets equal exactly if they contain identical elements.
+
+The library provides ways to define equalities on `Iterable`s, `List`s, `Set`s, and
+`Map`s, as well as combinations of these, such as:
+
+    const MapEquality(const IdentityEquality(), const ListEquality());
+
+This equality considers maps equal if they have identical keys, and the corresponding values are lists with equal (`operator==`) values.
+
+## Algorithms
+
+The algorithms library contains functions that operate on lists.
+
+It contains ways to shuffle a `List`, do binary search on a sorted `List`, and
+some different sorting algorithms.
+
+
+## Wrappers
+
+The wrappers library contains classes that "wrap" a collection.
+
+A wrapper class contains an object of the same type, and it forwards all
+methods to the wrapped object.
+
+Wrapper classes can be used in various ways, for example to restrict the type
+of an object to that of a supertype, or to change the behavior of selected
+functions on an existing object.