Add a mention to #24 in the README

This explains that immutable structures should only contain other
immutable structures when used against `.equal()`, or the result
is likely to be unexpected.

Author: astorije

README.md

@@ -62,6 +62,12 @@ var b = List.of(1, 2, 3);
 expect(a).to.equal(b);
 ```
 
+Immutable data structures should only contain other immutable data
+structures (unlike `Array`s and `Object`s) to be considered immutable and
+properly work against `.equal()`. See
+[this issue](https://github.com/astorije/chai-immutable/issues/24) for
+more information.
+
 ### .include(value)
 
 - **@param** *{ Mixed }* val

chai-immutable.js

@@ -74,6 +74,12 @@
      * expect(a).to.equal(b);
      * ```
      *
+     * Immutable data structures should only contain other immutable data
+     * structures (unlike `Array`s and `Object`s) to be considered immutable and
+     * properly work against `.equal()`. See
+     * [this issue](https://github.com/astorije/chai-immutable/issues/24) for
+     * more information.
+     *
      * @name equal
      * @alias equals
      * @alias eq