Add detail comments on valid_list in SimpleEncode

Change-Id: Iad7ca261a99c7b5f082cf3cc6504f4af438bf409
diff --git a/vp9/simple_encode.h b/vp9/simple_encode.h
index e021458..4221a70 100644
--- a/vp9/simple_encode.h
+++ b/vp9/simple_encode.h
@@ -83,6 +83,7 @@
   // kRefFrameTypeLast > kRefFrameTypePast > kRefFrameTypeFuture.
   // For example, if kRefFrameTypeLast and kRefFrameTypePast both point to the
   // same frame, kRefFrameTypePast will be set to invalid.
+  // 1: the ref frame type is available 0: the ref frame type is not available
   int valid_list[kRefFrameTypeMax];
 };