common: add zip command support

Signed-off-by: Lei Wen <leiwen@marvell.com>
diff --git a/README b/README
index a745d0b..af76b0c 100644
--- a/README
+++ b/README
@@ -1483,6 +1483,12 @@
 		can be displayed via the splashscreen support or the
 		bmp command.
 
+- Do compresssing for memory range:
+		CONFIG_CMD_ZIP
+
+		If this option is set, it would use zlib deflate method
+		to compress the specified memory at its best effort.
+
 - Compression support:
 		CONFIG_BZIP2