Added dynamic notification at top of site

* closes #4

Signed-off-by: Jim Martens <github@2martens.de>

_data/notification.yml

@@ -0,0 +1,5 @@
+show: false
+type: success
+text: >
+  Welcome on my relaunched website. Please see info on
+  <a class="alert-link" href="/about">About page</a>.

_includes/header.html

@@ -61,9 +61,10 @@
     {% endfor %}
 </header>
 
+{% if site.data.notification.show %}
 <div class="container my-3">
-    {% comment %}notification area {% endcomment %}
-    <div class="alert alert-primary" role="alert">
-        This is a primary alert—check it out!
+    <div class="alert alert-{{ site.data.notification.type }}" role="alert">
+        {{ site.data.notification.text }}
     </div>
 </div>
+{% endif %}