Copying the filesystem to the mounted NFS directory:

cp -axv /. /path/to/nfs/mountpoint/.
cp -axv /dev/. /path/to/nfs/mountpoint/dev/.

In /boot/cmdline.txt:

root=/dev/nfs rootfstype=nfs nfsroot=<nfsserver_ip_or_hostname>:<path_of_mountpoint> ip=::::<raspi_hostname>:eth0:dhcp