docs/web: give a little bit more exposure to our Google+ page

Add a small icon that links the Buildroot home page to our Google+
page.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

docs/header.html

@@ -57,7 +57,9 @@
     <br><a href="http://autobuild.buildroot.org">Autobuilder</a>
     <br><a href="http://patchwork.buildroot.org">Patchwork</a>
     <br><a href="/sponsors.html">Sponsors</a>
-
+    <br><a href="https://plus.google.com/100084591154899263420?prsrc=3" rel="publisher" target="_top" style="text-decoration:none;">
+      <img src="https://ssl.gstatic.com/images/icons/gplus-32.png" alt="Google+" style="border:0;width:32px;height:32px;"/>
+    </a>
 </td>