Explain compound arguments in README

[vladfaust]

That's a big thing, really. Having a key like "users:1", you don't have to build new Strings over and over. With MiniRedis, you can use compound arguments like this:

mini_redis.send("GET", {"users:", 1})

This code writes directly into IO, improving performance and efficiency drastically. Both single and compound keys can be Strings, Chars, Bytes and Numbers.