Add docs for -MD.

Change-Id: Iab83e3da183f1cb896d2058955338b328ca941a2
diff --git a/RSCCOptions.td b/RSCCOptions.td
index 76bb497..709e1a5 100644
--- a/RSCCOptions.td
+++ b/RSCCOptions.td
@@ -98,7 +98,7 @@
 def M_Group : OptionGroup<"M group>">;
 let Group = M_Group in {
 
-  def MD : Flag<["-"], "MD">;
+  def MD : Flag<["-"], "MD">, HelpText<"Emit .d dependency files">;
 
   def M : Flag<["-"], "M">;
   def emit_dep : Flag<["-"], "emit-dep">, Alias<M>;