Help: Add notes for topic 'ExternalProject-no-download-progress'

Help/release/dev/ExternalProject-no-download-progress.rst

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+ExternalProject-no-download-progress
+------------------------------------
+
+* The :module:`ExternalProject` module ``ExternalProject_Add`` command
+  learned a new ``DOWNLOAD_NO_PROGRESS`` option to disable progress
+  output while downloading the source tarball.