See http://plausiblethought.net/using-shortcodes-in-wordpress/ for details.

functions.php

<?php

function shortcodeButton( $atts, $content = null ) {  
	return '<a href="' . get_bloginfo('url') . '/contact/" class="btn">' . $content .' </a>';  
} 

add_shortcode('button', 'shortcodeButton');