Add documentation regarding how to build Ruby for debugging.

[ioquatix]

Is this the best way @nobu? Do you have any other advice?

[ioquatix]

@nobu do you think --enable-debug-env should set -O0 etc?

[nobu]

Hmmm, now optflags is defaulted to -O3.
I think it's ok to replace the default.

[nobu]

now optflags is defaulted to -O3.

if the compiler is gcc-variant.

[nobu]

Ah, --enable-debug-env?
It is to "enable RUBY_DEBUG environment variable", different purpose.

[ioquatix]

Thanks @nobu - I believe documentation like this is really helpful. If you have any other ideas let's discuss it.