fix readme path change

setup.py

 from setuptools import setup, find_packages
 
 here = os.path.abspath(os.path.dirname(__file__))
-with open(os.path.join(here, 'README.txt')) as f:
+with open(os.path.join(here, 'README.md')) as f:
     README = f.read()
 with open(os.path.join(here, 'CHANGES.txt')) as f:
     CHANGES = f.read()