doc: document the fdtdir PXE command

Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>

doc/README.pxe

@@ -155,6 +155,13 @@ fdt <path>	    - if this label is chosen, use tftp to retrieve the fdt blob
 		      the fdt_addr_r environment variable, and that address will
 		      be passed to bootm.
 
+fdtdir <path>	    - if this label is chosen, use tftp to retrieve a fdt blob
+		      relative to <path>. If the fdtfile environment variable
+		      is set, <path>/<fdtfile> is retrieved. Otherwise, the
+		      filename is generated from the soc and board environment
+		      variables, i.e. <path>/<soc>-<board>.dtb is retrieved.
+		      If the fdt command is specified, fdtdir is ignored.
+
 localboot <flag>    - Run the command defined by "localcmd" in the environment.
 		      <flag> is ignored and is only here to match the syntax of
 		      PXELINUX config files.