Create the initrd (i.e. initramfs-kernel-version.img) with network support:
dracut initramfs-kernel-version.img vmlinuz-kernel-version
Copy the resulting initramfs-kernel-version.img into the tftp boot directory as well.
Edit the default boot configuration to use the initrd and kernel inside /var/lib/tftpboot. This configuration should instruct the diskless client's root to mount the exported file system (/exported/root/directory) as read-write. To do this, configure /var/lib/tftpboot/pxelinux.cfg/default with the following:

default rhel6

label rhel6
  kernel vmlinuz-kernel-version
  append initrd=initramfs-kernel-version.img root=nfs:server-ip:/exported/root/directory rw