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

functions.php

<?php

function shortcodeButton( $atts, $content = null ) {
 	 extract( shortcode_atts( array(
		'link' => 'contact/',
		'colour' => 'green'
	), $atts ) );

	return '<a href="' . "{$link}" . '" class="btn btn-' . "{$colour}" . '">' . $content . ' </a>';  
} 

add_shortcode('button', 'shortcodeButton');