add notes how to start Plack with hot-reloading

Add notes to readme explaining how to start Koha/Plack with hot-reloading.

Basically what I described here: https://bugs.koha-community.org/bugzilla3/show_bug.cgi?id=31729#c54

This would also sort-of fix #307 (closed) though only via some manual work. To actually add an hot-reload flag, we would need to either change koha-plack (which would introduce some risk of people running a hot-reload enabled Plack in production); or change ktd to NOT use the debian wrapper scripts but plackup directly.