Document methods

Author: dgraham

README.md

@@ -1,6 +1,8 @@
 # MultiMap
 
-Store multiple values under each map key.
+The [multimap][1] data structure is a map in which more than one value may be stored under each key.
+
+[1]: https://en.wikipedia.org/wiki/Multimap
 
 ## Installation
 
@@ -19,8 +21,27 @@ const map = new MultiMap<string, number>()
 map.set('a', 1)
 map.set('a', 2)
 map.get('a') // => Set([1, 2])
+map.has('a') // => true
+map.size // => 1
 ```
 
+### Methods
+
+- `get(key)` - Retrieve the Set of values stored under a key or the empty Set if the key does not exist.
+- `set(key, value)` - Add a value to the key's set without removing previous values.
+- `has(key)` - Returns true if a value is stored under the key.
+- `delete(key)` - Remove key and all of key's values.
+- `delete(key, value)` - Remove a value from the key's set.
+- `drain(value)` - Remove a value from all keys that reference it.
+- `clear()` - Remove all keys and values to empty the map.
+- `keys()` - An iterator of map keys.
+- `values()` - An iterator of Sets of values for all keys.
+- `entries()` - An iterator of [key, Set<value>] pairs.
+
+### Properties
+
+- `size` - The number of keys in the map.
+
 ## Development
 
 ```