FindGDAL: add release note

Help/release/3.20.rst

@@ -186,6 +186,12 @@ Modules
   toolkits when ``nvcc`` is a symbolic link,
   for example due to a ``ccache`` or ``colornvcc`` wrapper script.
 
+* The :module:`FindGDAL` module has been improved to document and mark as
+  advanced its cache variables. There is a new ``FindGDAL_SKIP_GDAL_CONFIG``
+  variable which may be used to skip over the ``gdal-config``-based search.
+  Users may also set ``GDAL_ADDITIONAL_LIBRARY_VERSIONS`` to add additional
+  versions to the library name search strategy.
+
 * The :module:`FindIntl` module now provides an imported target.
 
 * The :module:`FindOpenSSL` module learned to support a version range.