Removed seventh-packet-byte-count.png from .gitignore

and added a note in seventh.cc file/tutorial

.gitignore

@@ -16,7 +16,6 @@
 *.routes
 *.tr
 [D|U]l[A-Z][a-z]*Stats.txt
-seventh-packet-byte-count.png
 
 \#*#
 ~*

examples/tutorial/seventh.cc

@@ -58,6 +58,9 @@ NS_LOG_COMPONENT_DEFINE ("SeventhScriptExample");
 // So first, we create a socket and do the trace connect on it; then we pass
 // this socket into the constructor of our simple application which we then
 // install in the source node.
+//
+// NOTE: If this example gets modified, do not forget to update the .png figure
+// in src/stats/docs/seventh-packet-byte-count.png
 // ===========================================================================
 //
 class MyApp : public Application