Document HARNESS_PERL_SWITCHES.

Mention a few handy uses.

Also bring up that -w is on by default and how to work around it.  -X doesn't work
to reverse -w because it turns off all warnings, even with "use warnings" on.
Merge pull request #10 from Perl-Toolchain-Gang/docs/HARNESS_PERL_SWITCHES

Document HARNESS_PERL_SWITCHES