add link about using 4.x kernel

doc/fai-guide.txt

@@ -5,7 +5,7 @@
 FAI Guide (Fully Automatic Installation)
 ========================================
 Thomas Lange <lange@informatik.uni-koeln.de>
-Sun, 22 Nov 2015
+Sun, 23 Jun 2016
 :Date: a date
 :Revision: 5.0
 
@@ -2205,10 +2205,11 @@ name of your package to /etc/fai/NFSROOT. Then call
 
 - Can I use a 4.X kernel?
 +
-Currently FAI has no support for a 4.x kernel, because aufs file
-system support was replaced by overlayfs. But overlayfs is not
-working when using NFS. FAI will switch to overlayfs if the NFS
-problems are solved.
+Yes. Have a look at
+https://lists.uni-koeln.de/pipermail/linux-fai/2016-March/011283.html
+
+FAI will switch to overlayfs in the near future, since the NFS
+problems are now solved.
 
 - How to use the nfsrooot as system for diskless clients?
 +