doc: clarify graph view docs to note negative values appear in profile comparison (google#667)

* doc: clarify graph view docs to note negative values appear in profile comparison.

Fixes google#666

* 80 char col

Author: nolanmar511

doc/README.md

@@ -192,7 +192,9 @@ the graph, controlled by the *nodecount* option.
 
 * **Node Color**:
   * large positive cum values are red.
-  * large negative cum values are green.
+  * large negative cum values are green; negative values are most likely to
+    appear during profile comparison, see [this section](#comparing-profiles)
+    for details.
   * cum values close to zero are grey.
 
 * **Node Font Size**: