Update readme and example image

Author: milliams

README.rst

@@ -41,7 +41,9 @@ For example, code like:
             .add(&s1)
             .add(&s2)
             .x_range(-5., 10.)
-            .y_range(-2., 6.);
+            .y_range(-2., 6.)
+            .x_label("Some varying variable")
+            .y_label("The response of something");
 
         // A page with a single view is then saved to an SVG file
         Page::single(&v).save("scatter.svg");