Refer to cross-build script in README

Change-Id: Ic983d2505e40891ba0d0eea3468a71a7741f5c3a
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-on: http://openocd.zylin.com/5711
Tested-by: jenkins
Reviewed-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>

README

@@ -284,6 +284,10 @@ libraries that OpenOCD depends on. Alternatively, you can specify
 *_CFLAGS and *_LIBS environment variables directly, see "./configure
 --help" for the details.
 
+For a more or less complete script that does all this for you, see
+
+  contrib/cross-build.sh
+
 Parallel Port Dongles
 ---------------------