update helper.rst to explain new-style config

fixes #1915

I meant to commit this to github.com/zooko/tahoe-lafs and pull-request to github.com/tahoe-lafs/tahoe-lafs, but instead I committed it to github.com/tahoe-lafs/tahoe-lafs, so I'm pull-requesting it to github.com/warner/tahoe-lafs to get Brian's attention.