Add simple message to explain active development to main page

Signed-off-by: Phil Peble <[email protected]>

content/en/_index.html

@@ -18,6 +18,17 @@ <h2 class="mt-4">
 </div>
 {{< /blocks/cover >}}
 
+{{% blocks/lead color="red" %}}
+  <h2>Heads up!</h2>
+
+  <p>
+    This site is under very active development as we migrate documentation here.
+    If you can't find what you're looking for, check out the
+    <span style="font-family: monospace; font-weight: bold;">#emissary-ingress</span>
+    channel in the <a href="https://slack.cncf.io/" style="color: white;">CNCF Slack</a>.
+  </p>
+{{% /blocks/lead %}}
+
 {{% blocks/lead color="white" %}}
   <h1>What is Emissary-ingress?</h1>
   <p>