Added a note to the dom-crawler docs about character sets

I added a note to the documentation to make it clear that when adding content using the default examples, the charset is assumed to be ISO-8859-1. If one wants to crawl/scrape UTF-8 documents, they should create a blank object and use addHTMLContent and specify the character set.

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.0+
| Fixed tickets | n/a