README: document CONFIG_BOARD_SIZE_LIMIT

CONFIG_BOARD_SIZE_LIMIT was introduced by f3a14d37 [Makefile: allow
boards to check file size limits] and is in use by several boards, but
never got documented.

Signed-off-by: Sascha Silbe <t-uboot@infra-silbe.de>

README

@@ -4371,6 +4371,11 @@ use the "saveenv" command to store a valid environment.
 		later, once stdio is running and output goes to the LCD, if
 		present.
 
+- CONFIG_BOARD_SIZE_LIMIT:
+		Maximum size of the U-Boot image. When defined, the
+		build system checks that the actual size does not
+		exceed it.
+
 Low Level (hardware related) configuration options:
 ---------------------------------------------------