Add template file for Gretl (#3737)

Co-authored-by: Nbl-ue <54780737+bl-ue@users.noreply.github.com>

community/Gretl.gitignore

+# gitignore template for Gretl
+# website: http://gretl.sourceforge.net/
+
+# Auto-generated log file is overwritten whenever you start a new session
+session.inp
+
+# Auto-generated temporary string code table
+string_table.txt