Update README.

Author: mbostock

README.md

@@ -273,7 +273,7 @@ Returns the value for the specified *key* string. If the map does not have an en
 
 <a name="map_set" href="#map_set">#</a> <i>map</i>.<b>set</b>(<i>key</i>, <i>value</i>)
 
-Sets the *value* for the specified *key* string; returns the new *value*. If the map previously had an entry for the same *key* string, the old entry is replaced with the new value.
+Sets the *value* for the specified *key* string. If the map previously had an entry for the same *key* string, the old entry is replaced with the new value. Returns the map, allowing chaining.
 
 <a name="map_remove" href="#map_remove">#</a> <i>map</i>.<b>remove</b>(<i>key</i>)
 
@@ -326,7 +326,7 @@ Returns true if and only if this set has an entry for the specified *value* stri
 
 <a name="set_add" href="#set_add">#</a> <i>set</i>.<b>add</b>(<i>value</i>)
 
-Adds the specified *value* string to this set. Returns *value*.
+Adds the specified *value* string to this set. Returns the set, allowing chaining.
 
 <a name="set_remove" href="#set_remove">#</a> <i>set</i>.<b>remove</b>(<i>value</i>)