Update sky.adoc

Added link to new article on skymap creation.

src/docs/asciidoc/jme3/advanced/sky.adoc

@@ -59,3 +59,4 @@ Box or Sphere?
 *  If you have access to sphere map textures – specially projected sky images that fit inside a sphere – then you use a SkySphere or SkyDome. 
 **  link:http://wiki.delphigl.com/index.php/Datei:Skysphere.jpg[SkySphere example]
 
+See <<jme3/advanced/free_skymaps#,How to create free skymaps>> for more information on Skymap creation.