add doc for functional

docs/functional.rst

@@ -0,0 +1,20 @@
+Functional
+----------
+
+.. py:function:: pydu.functional.compose(*funcs)
+
+    Compose all functions. The previous function must accept one argument,
+    which is the output of the next function. The last function can accept
+    any args and kwargs.
+
+    compose(f1, f2, f3)(*x) is same to f1(f2(f3(*x))).
+
+    >>> from pydu.functional import compose
+    >>> def f1(a):
+    ...     return a+1
+    ...
+    >>> def f2(a, b=2):
+    ...     return a+b
+    ...
+    >>> compose(f1, f2)(1, b=3)
+    5

docs/index.rst

@@ -28,6 +28,7 @@ Content
    dict
    environ
    exception
+   functional
    inspect
    iter
    list