Fix sinter example code in README.md

[jungmin-cho-eng]

This applies the second recommendation in #20.

Without wrapping the body of the sinter example code with a conditional if __name__ == "__main__":, the code produces the following error:

AttributeError: 'NoneType' object has no attribute 'kill'

According to the sinter documentation, this error occurs because the multiprocessing children spawned by sinter.collect also attempt to execute the top-level code.

# NOTE: This is actually necessary! If the code inside 'main()' was at the
# module level, the multiprocessing children spawned by sinter.collect would
# also attempt to run that code.
if __name__ == '__main__':
    main()

[trmue]

Thanks!