Fix missing README file in source distribution

I've got error while trying install ``melody-dl`` from PyPI,

```
# pip install melody-dl
Downloading/unpacking melody-dl
  Downloading melody-dl-0.1.5.tar.gz
  Running setup.py (path:/tmp/pip_build_root/melody-dl/setup.py) egg_info for package melody-dl
    Traceback (most recent call last):
      File "<string>", line 17, in <module>
      File "/tmp/pip_build_root/melody-dl/setup.py", line 17, in <module>
        long_description=open('README.md').read(),
    IOError: [Errno 2] No such file or directory: 'README.md'
    Complete output from command python setup.py egg_info:
    Traceback (most recent call last):

  File "<string>", line 17, in <module>

  File "/tmp/pip_build_root/melody-dl/setup.py", line 17, in <module>

    long_description=open('README.md').read(),

IOError: [Errno 2] No such file or directory: 'README.md'

----------------------------------------
Cleaning up...
Command python setup.py egg_info failed with error code 1 in /tmp/pip_build_root/melody-dl
Storing debug log for failure in /root/.pip/pip.log
```

Problem caused by missing ``README.md`` file in ``melody-dl-0.1.5.tar.gz`` archive.