README file improvements

[Markdown](http://en.wikipedia.org/wiki/Markdown) documents looks much better than simple plain text on GitHub. So I've ported original [README.txt](https://github.com/BrendanMulcahy/Dota-2-Alt-Tab/blob/2c0cf7b676fff4f0bb8c9c98c8ba6f3e9c34da23/README.txt) to [README.md](https://github.com/Tsukanov/Dota-2-Alt-Tab/blob/cd094bef4b187f14276c7a66b3dc17c33a810210/README.md).
Also, fixed some typos.