email regexp should have a source comment explaining the rationale

[mk-pmb]

Currently in line 417 of core.js.

First, big thumbs up :+1: that it looks like this module won't claim email addresses with a "+" in the name as invalid.

That said, since the current regexp is very permissive, and more accurate (probably slower) regexps have been invented and discussed, the source should carry a comment with a link explaining this project's stance on what to expect from an email regexp.