README: document and quantify the difference between NBench and Perl
README: document and quantify the difference between NBench and Perl

While at it, also show how Perl's perf is very similar to SPEC06's perlbench.

Signed-off-by: Emilio G. Cota <201e881bfa0e9543bba00630d033ebfb953e7c40@braap.org>

cota/dbt-bench