[TOOLS] add a README to the dtc directory.

These are instructions to be used to upgrade the "Device Tree
Compiler" in Xvisor

Signed-off-by: Anup Patel <anup@brainfault.org>

tools/dtc/README

@@ -0,0 +1,15 @@
+This directory contains the source tree of the "Device tree Compiler" used
+for Xvisor development.
+
+This is a copy of the "Device tree compiler" directory of the linux 3.x
+source tree.
+
+In the linux source tree the "Device tree compiler" directory is at 
+scripts/dtc.
+
+Anytime you want to upgrade the Xvisor DTC compiler you should replace
+all files in this directory with the latest from the linux source tree.
+
+You should preserve only 2 files:
+* The Makefile
+* This README