add dictionary documentation

docs/lang.rst

@@ -55,6 +55,37 @@ Libraries
 
   Delimited control operators.
 
+- ``(picrin dictionary)``
+
+  Symbol to Object table. Internally it is implemented on hash-table.
+
+  Note that dictionary is not a weak map; if you are going to make a highly memory-consuming program with dictionaries, you should know that dictionaries keep their bound objects and never let them free until you explicitly deletes bindings.
+
+  - ``(dictionary)``
+
+    Returns a newly allocated empty dictionary. In the future, it is planned to extend this function to take optional arguments for initial key/values.
+
+  - ``(dictionary? obj)``
+
+    Returns ``#t`` if obj is a dictionary.
+
+  - ``(dictionary-ref dict key)``
+
+    Look up dictionary dict for a value associated with symbol key. If no object is associated with key, it will raise an error.
+
+  - ``(dictionary-set! dict key obj)``
+
+    If there is no value already associated with key, this function newly creates a binding of key with obj. Otherwise, updates the existing binding with given obj.
+
+  - ``(dictionary-delete dict key)``
+
+    Deletes the binding associated with key from dict. If no binding on dict is associated with key, an error will be raised.
+
+
+  - ``(dictionary-size dict)``
+
+    Returns the number of registered elements in dict.
+
 - ``(picrin user)``
 
   When you start the REPL, you are dropped into here.