Add documentation for XRegExp.tag (#206)
Add documentation for XRegExp.tag (#206)

* Add documentation for XRegExp.tag

Adapted from the example in https://github.com/slevithan/xregexp/pull/205#issuecomment-354236652

* Add example to XRegExp.tag docs

This was copied from the source code comment.

GerHobbelt/xregexp,mathiasbynens/xregexp,GerHobbelt/xregexp,GerHobbelt/xregexp,slevithan/XRegExp,slevithan/XRegExp,slevithan/xregexp,slevithan/xregexp,mathiasbynens/xregexp,slevithan/xregexp,mathiasbynens/xregexp,mathiasbynens/xregexp