Add release notes.

Author: waylan

docs/change_log/release-3.3.md

@@ -66,6 +66,21 @@ The following new features have been included in the 3.3 release:
  Any random HTML attribute can be defined and set on the `<code>` tag of fenced code
  blocks when the `attr_list` extension is enabled (#816).
 
+* The HTML parser has been completely replaced. The new HTML parser is built on Python's
+ [html.parser.HTMLParser](https://docs.python.org/3/library/html.parser.html), which
+ alleviates various bugs and simplify maintenance of the code (#803, #830).
+
+* The [Markdown in HTML](../md_in_html.md) extension has been rebuilt on the new HTML
+ Parser, which drastically simplifies it. Note that raw HTML elements with a `markdown`
+ attribute defined are now converted to ElementTree Elements and are rendered by the
+ serializer. Various bugs have been fixed (#803, #595, #780, and #1012).
+
+* Link reference parsing, abbreviation reference parsing and footnote reference parsing
+ has all been moved from `preprocessors` to `blockprocessors`, which allows them to be
+ nested within other block level elements. Specifically, this change was necessary to
+ maintain the current behavior in the rebuilt Markdown in HTML extension. A few random
+ edge-case bugs (see the included tests) were resolved in the process (#803).
+
 ## Bug fixes
 
 The following bug fixes are included in the 3.3 release: