Added instruction for initial setup of depend file

Author: hsbt

tool/update-deps

@@ -17,6 +17,14 @@
 #   3. Use --fix to fix makefiles.
 #      Ex.  ./ruby tool/update-deps --fix
 #
+# Usage to create a depend file initially:
+#   1. Copy the dependency section from the Makefile generated by extconf.rb.
+#      Ex.  ext/cgi/escape/Makefile
+#   2. Add `# AUTOGENERATED DEPENDENCIES START` and `# AUTOGENERATED DEPENDENCIES END`
+#      sections to top and end of the depend file.
+#   3. Run tool/update-deps --fix to fix the depend file.
+#   4. Commit the depend file.
+#
 # Other usages:
 #   *  Fix makefiles using previously detected dependency problems
 #      Ex.  ruby tool/update-deps --actual-fix [file]