add doc for environ.environ

docs/environ.rst

@@ -0,0 +1,17 @@
+Environ
+-------
+
+
+.. py:function:: pydu.environ.environ(**kwargs)
+
+    Context manager for updating one or more environment variables.
+
+    Preserves the previous environment variable (if available) and
+    recovers when exiting the context manager.
+
+    >>> from pydu.environ import environ
+    >>> with environ(a='a'):
+    ...     print(os.environ['a'])
+    ...
+    a
+

docs/index.rst

@@ -26,6 +26,7 @@ Content
    console
    convert
    dict
+   environ
    exception
    inspect
    list