log: Add units to code-size stats in README.log

Without the units these numbers are confusing. Add a comment about the
unit being 'bytes' and mention 'buildman' as the source.

Suggested-by: Lukasz Majewski <lukma@denx.de>
Signed-off-by: Simon Glass <sjg@chromium.org>

doc/README.log

@@ -162,7 +162,8 @@ Code size
 ---------
 
 Code size impact depends largely on what is enabled. The following numbers are
-for snow, which is a Thumb-2 board:
+generated by 'buildman -S' for snow, which is a Thumb-2 board (all units in
+bytes):
 
 This series: adds bss +20.0 data +4.0 rodata +4.0 text +44.0
 CONFIG_LOG: bss -52.0 data +92.0 rodata -635.0 text +1048.0