Update release notes

docs/pages/release_notes.md

@@ -18,6 +18,29 @@ This is a {{ site.pmd.release_type }} release.
 
 ### API Changes
 
+#### Deprecated API
+
+Some API deprecations were performed in core PMD classes, to improve compatibility with PMD 7.
+- {% jdoc core::Report %}: construction methods like addViolation or createReport
+- {% jdoc core::RuleContext %}: all constructors, getters and setters. A new set
+of stable methods, matching those in PMD 7, was added to replace the `addViolation`
+overloads of {% jdoc core::lang.rule.AbstractRule %}. In PMD 7, `RuleContext` will
+be the API to report violations, and it can already be used as such in PMD 6.
+- {% jdoc core::RuleSet %}: methods that serve to apply rules, including `apply`, `start`, `end`, `removeDysfunctionalRules`
+
+#### Changed API
+
+It is now forbidden to report a violation:
+- With a `null` node
+- With a `null` message
+- With a `null` set of format arguments (prefer a zero-length array)
+
+{% jdoc core::RuleContext %} now requires setting the current rule before calling
+{% jdoc core::lang.rule.Rule#apply(java.util.List, core::RuleContext)}. This is
+done automatically by `RuleSet#apply` and such. Creating and configuring a
+`RuleContext` manually is strongly advised against, as the lifecycle of `RuleContext`
+will change drastically in PMD 7.
+
 ### External Contributions
 
 {% endtocmaker %}