add section about nfsroot and NFS v4

doc/fai-guide.txt

@@ -2203,6 +2203,27 @@ should be performed, or if the client should to boot from local
 disk. This is done using `fai-chboot(8)`.
 
 
+=== [[nfsv4]]How to use NFS v4 with the nfsroot
+
+The nfsroot is mounted via NFS v3 by default. This is done
+by adding the option 'nfsvers=3' to the kernel boot options of the
+kernel booted by the install clients. fai-chboot(8) will do this for you.
+
+If you want to use NFS v4 for the nfsroot, you have to remove this
+option the the kernel boot options. Additionally, overlayfs
+- the kernel module that is used for making the read-only mounted
+nfsroot writeable - can only work with NFS v4 if acl(5) support is
+disabled on the filessystem on the NFS server side (as of 2019). If
+'/srv/fai/nfsroot' on your FAI server is located on the root partition
+mounted to '/', you can disable acl support for the whole filesystem
+on this partition using
+
+----
+# mount -oremount,noacl /
+----
+
+Then mounting the nfsroot from a FAI server using NFS v4 will work.
+
 === [[debian-mirror]]How to create a local Debian mirror
 
 The utility `mkdebmirror` footnote:[You can find the script in