Add install section to README.md

I put some info about installation over Composer (from Packagist). Inpiration from others readme:
- https://github.com/chriswoodford/foursquare-php
- https://github.com/jcroll/foursquare-api-client
- https://github.com/nette/tracy
- https://github.com/squizlabs/PHP_CodeSniffer