add doc for path

docs/index.rst

@@ -30,6 +30,7 @@ Content
    list
    misc
    network
+   path
    request
    set
    string

docs/path.rst

@@ -0,0 +1,10 @@
+Path
+----
+
+.. py:function:: pydu.path.cd(path)
+
+    Context manager for cd the given path.
+
+    >>> from pydu.path import cd
+    >>> with cd('test'):
+    ...     pass