Documentation update

markdown.md

 
 ### HTML Tags
 
-Currently all HTML tags and attributes are permitted, when markdown syntax doesn't do what you want.
+HTML tags can be used verbatim where markdown is too limiting.
+
+The allowable tags, attributes, and CSS are governed by `Markdown.tagFilter`
+which is a configurable `MDHTMLFilter` object. Anything not excplicitly included
+in the filter is rendered as plain text or stripped from the output. For example,
+Javascript event hooks are disallowed, as are scripts, stylesheets, and iframes,
+among many others.
 
 Class:
 * `MDHTMLTagReader`