Exclude a failing test in MingW build.

python/README.txt

diff --git a/python/README.txt b/python/README.txt
index 6dc144e..bb0032f 100644
--- a/python/README.txt
+++ b/python/README.txt
@@ -43,8 +43,9 @@
 
      $ protoc --version
 
-4) Run the tests:
+4) Build and run the tests:
 
+     $ python setup.py build
      $ python setup.py test
 
    If some tests fail, this library may not work correctly on your