Making UTF-8 encoding explicit when reading README.md

[ccrndn]

"pip install causalml" fails (decoding error) when the system encoding is not UTF8 (e.g ASCII).

Complete output from command python setup.py egg_info:
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
      File "/tmp/pip-build-ccs_s93d/causalml/setup.py", line 17, in <module>
        long_description = f.read()
      File "/usr/lib64/python3.6/encodings/ascii.py", line 26, in decode
        return codecs.ascii_decode(input, self.errors)[0]
    UnicodeDecodeError: 'ascii' codec can't decode byte 0xc3 in position 8370: ordinal not in range(128)

[CLAassistant]

All committers have signed the CLA.

[jeongyoonlee]

Thanks for the contribution, @ccrndn! Could you please sign the Contributor License Agreement so that we can accept your contribution? Thanks!

[jeongyoonlee]

LGTM. Thanks!