Readd `subject to change` for site/project prefixes.

Depending on how the user.js engine standard sways this will most likely need to change... let the authors know beforehand.

Refs:
* #226
* greasemonkey/greasemonkey#1963
* https://github.com/Ventero/greasemonkey/commit/41c0f273ae7da2c169e4582363483d5184408463#commitcomment-7166075 and forward