Finally added doc. Fixed #21

Author: Sylvain MARIE

docs/usage.rst

@@ -1,7 +1,10 @@
-Usage (Or: How To Get The List of Libraries)
-============================================
+Usage
+=====
 
-The primary function that you'll care about in this package is ``stdlib_list.stdlib_list``.
+Getting The List of Libraries
+-----------------------------
+
+``stdlib_list.stdlib_list`` returns the list of libraries in stdlib for any given version (by default, current python version).
 
 In particular:
 
@@ -15,5 +18,30 @@ In particular:
     Out[3]: ['__future__', '__main__', '_dummy_thread', '_thread', 'abc', 'aifc']
 
 
+Checking if a Module is part of stdlib
+--------------------------------------
+
+``stdlib_list.in_stdlib`` provides an efficient way to check if a module name is part of stdlib.
+It relies on ``@lru_cache`` to cache the stdlib list and query results for similar calls. Therefore it is much more efficient than ``module_name in stdlib_list()`` especially if you wish to perform multiple checks.
+
+In particular:
+
+::
+
+    >>> from stdlib_list import in_stdlib
+    >>> in_stdlib('zipimport')  # built in
+    True
+    >>> in_stdlib('math')       # C-API stdlib module, but linked as extension (on my machine)
+    True
+    >>> in_stdlib('numpy')      # C-API extension, not stdlib
+    False
+    >>> in_stdlib('sys')        # built-in (and special)
+    True
+    >>> in_stdlib('os')         # Python code in stdlib
+    True
+    >>> in_stdlib('requests')   # Python code, not stdlib
+    False
+
+
 .. automodule:: stdlib_list
-    :members: stdlib_list
+    :members: stdlib_list, in_stdlib