Add brief documentation for IntSet

README.rst

@@ -27,6 +27,7 @@ This package implements a variety of data structures, including
 * Trie
 * Linked List
 * Sorted Dict, Sorted Multi-Dict and Sorted Set
+* DataStructures.IntSet
 
 ------
 Deque
@@ -363,6 +364,21 @@ A list of sequentially linked nodes. This allows efficient insertion of nodes to
   julia> for i in l5; print(i); end
   246
 
+---------------------
+DataStructures.IntSet
+---------------------
+
+`DataStructures.IntSet` is a drop-in replacement for the Base `IntSet` type. It
+efficiently stores dense collections of small non-negative `Int`s as a sorted
+set. The constructor `IntSet([itr])` constructs a sorted set of the integers
+generated by the given iterable object, or an empty set if no argument is
+given. If the set will be sparse (for example holding a few very large
+integers), use `Set` or `SortedSet` instead.
+
+A complement IntSet may be constructed with `complement` or `complement!`. The
+complement of an empty `IntSet` contains `typemax(Int)` elements from 0 to
+`typemax(Int)-1`.
+
 ----------------------------------------
 Overview of Sorted Containers
 ----------------------------------------