Ruby: add changenote for XXE improvements

Author: nickrolfe

ruby/ql/src/change-notes/2022-09-27-libxml-xxe.md

@@ -0,0 +1,6 @@
+---
+category: minorAnalysis
+---
+* The `rb/xxe` query has been updated to add the following sinks for XML external entity expansion:
+    1. Calls to parse XML using `LibXML` when its `default_substitute_entities` option is enabled.
+    2. Uses of the Rails methods `ActiveSupport::XmlMini.parse`, `Hash.from_xml`, and `Hash.from_trusted_xml` when `ActiveSupport::XmlMini` is configured to use `LibXML` as its backend, and its `default_substitute_entities` option is enabled.