Create release notes for WB 1.13.0 & 1.14.0

Document all known incompatibilities or breaking changes that may affect
user code.

RELEASE_NOTES.md

@@ -0,0 +1,28 @@
+# Eclipse WindowBuilder: Migration Guide
+
+This page describes all changes introduced by each release which may impact user
+code.
+
+## 1.14.0 (2023-12)
+
+### Deprecation of SWTResourceManager
+
+WindowBuilder now uses class-local instances of LocalResourceManager, instead of
+the global SWTResourceManager, to keep track of SWT resources.
+
+Note that WindowBuilder is still able to recognize usage of the
+SWTResourceManager in user code, but any further modifications will exclusively
+use the LocalResourceManager.
+
+Users can migrate their code by manually creating a local LocalResourceManager
+variable, by initializing it with `JFaceResources.managerFor(...)` and by then
+replacing all calls to ASTResourceManager with calls to the local variable.
+
+## 1.13.0 (2023-09)
+
+### Java 8 no longer supported
+
+It is no longer possible to use the Design editor with a Java 8 JRE. If your
+application requires to compile against this Java version, consider upgrading to
+the latest JDK and instead simply setting an appropriate project-specific
+compiler compliance.