Add the CR logo instead of the text

Author: vgvassilev

_includes/header.html

@@ -7,8 +7,9 @@
         <span class="icon-bar"></span>
         <span class="icon-bar"></span>
       </button>
-      
-    <a class="navbar-brand" href="{{ site.url }}{{ site.baseurl }}/">Compiler Research</a>
+      <a href="{{ site.url }}{{ site.baseurl }}">
+        <img style="margin:0;" width="48px" src="/images/cr-logo.png" />
+      </a>
     </div>
     <ul id="navbar-collapse-1" class="nav navbar-nav ml-auto navbar-collapse collapse">
       <li class="nav-item dropdown">