additions to streamtube doc

Author: emmanuelle

src/traces/streamtube/index.js

@@ -53,7 +53,9 @@ module.exports = {
             'By default, the tubes\' starting positions will be cut from the vector field\'s',
             'x-z plane at its minimum y value.',
             'To specify your own starting position, use attributes `starts.x`, `starts.y`',
-            'and `starts.z`.'
+            'and `starts.z`.',
+	    'The color is encoded by the norm of (u, v, w), and the local radius',
+	    'by the divergence of (u, v, w)'.
         ].join(' ')
     }
 };