Help: Add notes for topic 'ExternalData-missing-not-fatal'

Help/release/dev/ExternalData-missing-not-fatal.rst

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+ExternalData-missing-not-fatal
+------------------------------
+
+* The :module:`ExternalData` module learned to tolerate a ``DATA{}``
+  reference to a missing source file with a warning instead of
+  rejecting it with an error.  This helps developers write new
+  ``DATA{}`` references to test reference outputs that have not
+  yet been created.