Updated contributing notes

CONTRIBUTING.md

@@ -1,12 +1,46 @@
 # CONTRIBUTING
 
-Welcome to dash-canvas!
+Welcome to dash-canvas! Contributions, bug reports, comments about the
+package are very welcome. The project is hosted on
+https://github.com/plotly/dash-canvas. 
 
-## Structure of the code
+This project was initially generated by the
+[dash-component-boilerplate](https://github.com/plotly/dash-component-boilerplate),
+which contains the minimal set of code required to create your own custom
+Dash component.
 
-React component DashCanvas: `src/lib/components/DashCanvas.react.js`
+## Structure of the code:
 
-Build Javascript component: `npm run build:all` at the root (see https://dash.plot.ly/react-for-python-developers for more details).
+- the source code of the ``dash_canvas.DashCanvas`` canvas reactive
+  object is the JavaScript React class in
+``src/lib/components/DashCanvas.react.js``. If you are not familiar with
+React and JavaScript, [this
+tutorial](https://dash.plot.ly/react-for-python-developers) is a good
+primer. In order to build the Javascript component: execute at the root
+of the package
 
-Utility Python functions: `dash_canvas/utils`
+```
+npm run build:all
+```
 
+- the ``dash_canvas/utils`` submodule contains utility functions in order
+  to use annotations retrieved from the DashCanvas object for further
+image processing.
+
+- for examples of applications using dash-canvas, see the https://github.com/plotly/canvas-portal. These examples show in particular how to write callbacks using the DashCanvas object properties.
+
+## Testing
+
+For testing the DashCanvas objects using Selenium 
+
+```
+pytest tests --webdriver Chrome
+```
+
+(or using another webdriver).
+
+For testing the Python package and utility functions 
+
+```
+pytest dash_canvas
+```