Add a note to the software list template

Add a note that directs viewers of the generated SOFTWARE-LIST.md to
refer to CONTRIBUTING.md for directions on updating product
information.

Co-authored-by: dav3r <david.redmin@trio.dhs.gov>

config/SOFTWARE-LIST.tpl.md

@@ -17,4 +17,7 @@ This list was initially populated using information from the following sources:
 - Kevin Beaumont
 - SwitHak
 
+NOTE: This file is automatically generated. To submit updates, please refer to
+[`CONTRIBUTING.md`](CONTRIBUTING.md).
+
 {{software_markdown_table}}