Update header.html

Author: danielbachhuber

_includes/header.html

@@ -26,7 +26,7 @@
 	{% endif %}
 		<header class="cf">
 		<div class="container">
-			<h1><a href="/">WP-CLI</a></h1>
+			<h1><a href="/"><img alt="WP-CLI" src="/assets/img/wp-cli-logo.png" height="40px"></a></h1>
 			{% capture page_url %}{{ page.url | remove:'index.html' }}{% endcapture %}
 			{% if page_url == '/restful/' %}
 			<h2>The <em>RESTful</em> command line interface for WordPress</h2>