uncrustify: add some instructions

TOOLS/uncrustify.cfg

@@ -1,3 +1,10 @@
+# Usage:
+#    uncrustify -l C -c TOOLS/uncrustify.cfg --no-backup --replace file.c
+#
+# Keep in mind that this uncrustify configuration still produces some
+# bad/broken formatting.
+#
+
 code_width=80
 indent_align_string=false
 indent_braces=false