Help: Add notes for topic 'ExternalData-custom-download'

Help/release/dev/ExternalData-custom-download.rst

@@ -0,0 +1,7 @@
+ExternalData-custom-download
+----------------------------
+
+* The :module:`ExternalData` module learned to support
+  :ref:`Custom Fetch Scripts <ExternalData Custom Fetch Scripts>`.
+  This allows projects to specify custom ``.cmake`` scripts for
+  fetching data objects during the build.

Modules/ExternalData.cmake

@@ -253,6 +253,8 @@ The following hash algorithms are supported::
 Note that the hashes are used only for unique data identification and
 download verification.
 
+.. _`ExternalData Custom Fetch Scripts`:
+
 Custom Fetch Scripts
 ^^^^^^^^^^^^^^^^^^^^