Add readme caveat re URL escaping

[JakeQZ]

See #646.

Due to the way Emogrifier works internally by using DOMDocument, any lack of true replication of actual HTML markup when processed through loadHtml and then saveHtml will be evident in the output.

This includes

• URL-escaping of URL attributes, which may break templating engines.

Workarounds are possible, typically involving a str_replace before and after Emogrification.

This issue should be noted in the README.