MLBEDSW-3380 Update readme with build flags

mlw_codec is part of the codebase and has build flags.
README has been updated to include these.

Also, added -Werror to the list, as we must build without any warnings,
so treat warnings as errors.

Signed-off-by: Michael McGeagh <michael.mcgeagh@arm.com>
Change-Id: I10114bb013fad1ec1685fafc2e41c18ff12d9f9d
diff --git a/ethosu/mlw_codec/makefile b/ethosu/mlw_codec/makefile
index 6eb418d..68eb0d9 100644
--- a/ethosu/mlw_codec/makefile
+++ b/ethosu/mlw_codec/makefile
@@ -20,7 +20,7 @@
 
 UNAME=$(shell uname -o)
 
-CFLAGS=-Wall -Wno-unused-function -Wno-unused-variable
+CFLAGS=-Wall -Werror -Wno-unused-function -Wno-unused-variable
 
 ifeq ($(DEBUG),1)
     CFLAGS+=-g -O0 -DDEBUG