Fix "warning: no files found matching 'README.md'"

The path of README file is wrong, so when you run
`$ pip install bcdoc`, you encounter the warning message as follows:

Downloading/unpacking bcdoc>=0.12.0,<0.13.0 (from awscli)
  Downloading bcdoc-0.12.2.tar.gz
  Running setup.py (path:/tmp/pip_build_vagrant/bcdoc/setup.py) egg_info for package bcdoc

    warning: no files found matching 'README.md'