add sponsor display in website footer

docs/_data/sponsors.yml

@@ -0,0 +1,3 @@
+- name: Forestry.io
+  image: http://via.placeholder.com/200x100
+  url: https://forestry.io

docs/_includes/footer.html

@@ -11,5 +11,15 @@
         </a>
       </p>
     </div>
+    <div class="unit two-thirds align-right center-on-mobiles">
+      <p>
+        Sponsored by
+        {% for sponsor in site.data.sponsors %}
+          <a href="{{ sponsor.url }}">
+            <img src="{{ sponsor.image }}" width="100" height="30" alt="{{ sponsor.name }}">
+          </a>
+        {% endfor %}
+      </p>
+    </div>
   </div>
 </footer>

docs/_sass/_style.scss

@@ -224,8 +224,7 @@ footer {
 
   img {
     display: inline-block;
-    position: relative;
-    top: 8px;
+    vertical-align: middle;
     margin-left: 5px;
     opacity: .8;
     padding: 1px;