[jlreq] Pull Request: Add clarification to the introduction

frivoal has just submitted a new pull request for https://github.com/w3c/jlreq:

== Add clarification to the introduction ==
This leaves room for future extensions, manual overrides, and other
tweaks, while letting the document focus on the simple system without
getting into all sorts of corner cases and side discussions.

Closes #200

See https://github.com/w3c/jlreq/pull/204