Add tips for writing selection tests

Author: GlennMatthys

CONTRIBUTING.md

@@ -177,6 +177,32 @@ which shows the baseline image, the generated image, the diff and the json mocks
 
 To view the results of a run on CircleCI, download the `build/test_images/` and `build/test_images_diff/` artifacts into your local repo and then run `npm run start-image_viewer`.
 
+### Writing selection tests
+Keep in mind that the selection coordinates are relative to the top-left corner of the plot, including the margins. To produce a reliable selection test, 
+it may be necessary to to fix the width, height, margins, X axis range and Y axis range of the plot. For example:
+
+```
+Plotly.newPlot(gd,
+    [
+        {
+            x: [1, 1, 1, 2, 2, 2, 3, 3, 3],
+            y: [1, 2, 3, 1, 2, 3, 1, 2, 3],
+            mode: 'markers'
+        }
+    ],
+    {
+        width: 400, height: 400,
+        margin: {l: 100, r: 100, t: 100, b: 100},
+        xaxis: {range: [0, 4]},
+        yaxis: {range: [0, 4]}
+     }
+)
+```
+
+This will produce the following plot, and you want to simulate a selection path of (175, 175) to (225, 225):
+
+<img src="https://user-images.githubusercontent.com/31989842/38889542-5303cf9c-427f-11e8-93a2-16ce2e29f521.png">
+
 
 ## Repo organization