doc: update CHANGELOG with "notable changes" for v1.17

CHANGELOG.md

@@ -15,6 +15,34 @@ Nokogiri follows [Semantic Versioning](https://semver.org/), please see the [REA
 * [CRuby] Update to rake-compiler-dock v1.5.1 for building precompiled native gems. [#3216] @flavorjones
 
 
+### Notable changes
+
+#### SAX Parsers
+
+The XML and HTML4 SAX parsers have received a lot of attention in this release, and we've fixed multiple long-standing bugs with encoding and entity handling. In addition, libxml2 v2.13 has also made some underlying fixes and improvements to encoding and entity handling.
+
+We're shipping these fixes in a minor release because we firmly believe the resulting behavior is correct and standards-compliant, however applications that have been depending on the buggy behavior may be impacted.
+
+If your application relies on the SAX parsers, and in particular if you're SAX-parsing documents with parsed entities or incorrect encoding declarations, please read the changelog below carefully.
+
+
+#### Fragment parsing
+
+Document fragment parsing has been improved, particularly with respect to handling malformed fragments or fragments with implicit namespace prefixes. Namespace reconciliation still isn't where we want it to be, but it's an improvement.
+
+HTML5 fragment parsing now allows the context node to be specified as a keyword argument to the `HTML5::DocumentFragment.parse` and `.new` methods, which in particular should allow for more flexible sanitization and support for the [draft HTML Sanitizer API](https://wicg.github.io/sanitizer-api/) in downstream libraries.
+
+
+#### Error handling
+
+In scenarios where multiple errors could be reported by the underlying parser, the errors will be aggregated into a single `Nokogiri::XML::SyntaxError` that is raised. Previously only the final error reported by libxml2 was raised which was often misleading if it was only a warning and not the fatal error.
+
+
+#### Schema validation
+
+We've resolved many long-standing bugs in the various schema classes, validation methods, and their error reporting. Behavior is now consistent across schema types and input types, as well as parser backends (Xerces and libxml2).
+
+
 ### Added
 
 * Introduce support for a new SAX callback `XML::SAX::Document#reference`, which is called to report some parsed XML entities when `XML::SAX::ParserContext#replace_entities` is set to the default value `false`. This is necessary functionality for some applications that were previously relying on incorrect entity error reporting which has been fixed (see below). For more information, read the docs for `Nokogiri::XML::SAX::Document`. [#1926] @flavorjones