Add syntax highlighting to README.md

See the rich diff [here](https://github.com/ryanmjacobs/hiredis/commit/fc5f8d8d2553f914a7ed4fb7c6a1e2dcacac3c5e?short_path=04c6e90#diff-04c6e90faac2675aa89e2176d2eec7d8).