add a simple doctest example

doctest_test.py

@@ -0,0 +1,29 @@
+#!/usr/bin/python
+
+def foo(n):
+    """Returns n times foo, and a period.
+
+    Just some random doctest test.
+
+    Example:
+
+    >>> foo(1)
+    'foo.'
+
+    >>> foo(5)
+    'foo foo foo foo foo.'
+
+    >>> foo(0)
+    ''
+    """
+    if n >= 1:
+        return "foo " * (n-1) + "foo."
+    else:
+        return ""
+
+def _test():
+    import doctest
+    doctest.testmod()
+
+if __name__ == "__main__":
+    _test()