docs/about.html: add menuconfig screenshot

docs/about.html

@@ -5,6 +5,12 @@
 
 <h3>Buildroot: making Embedded Linux easy</h3>
 
+<div align="center">
+<a href="images/menuconfig.png">
+ <img src="images/menuconfig_small.png" width="340" height="220"
+ alt="Buildroot menuconfig" border="0">
+</a>
+</div>
 
 Buildroot is a set of Makefiles and patches that makes it easy to generate a
 cross-compilation toolchain and root filesystem for your target Linux