README.md: recommend -Og instead of -O0

README.md: recommend -Og instead of -O0

per https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html

> -Og
>  Optimize debugging experience. -Og should be the optimization level of
>  choice for the standard edit-compile-debug cycle, offering a reasonable
>  level of optimization while maintaining fast compilation and a good
>  debugging experience. It is a better choice than -O0 for producing
>  debuggable code because some compiler passes that collect debug
>  information are disabled at -O0.

so instead of using -O0, for better developer experience, -Og is
recommended.

Signed-off-by: Kefu Chai <kchai@redhat.com>