github_your_fork.user.js

// ==UserScript==
// @name        GitHub Your Fork
// @namespace   https://github.com
// @description Shows a 'forked to' button below the title
// @include     https://*github.com/*
// @require     http://ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js
// @version     1.0.4
// @grant       none
// ==/UserScript==

(function($) {

    var title = $('.repohead-details-container h1.public');
	if (title.length > 0 && $('h1 .fork-flag').length === 0) {
		var username = $('img.avatar').attr('alt').substring(1);
        var repository = title.find('[itemprop="name"] a').html();
		title.append('<span class="fork-flag" style="clear: both;"><span class="text">your fork is at <a href="/' + username + '/' + repository + '">' + username + '/' + repository + '</a></span></span>');
	}

})(jQuery);