Describe JS frontend usage.

README.md

@@ -46,7 +46,36 @@
 
 #### JavaScript client frontend
 
-TODO
+On startup, the user is presented with a full-screen map view. At that point, it
+is possible to zoom, pan and move around the map.
+Via a control in the top left corner, the creation / edition of user
+features can be made.
+
+A "feature" is made of several properties : an ID, a label and a geometry.
+While the ID is automatically generated by the server, the user is responsible
+of setting the value of the label (form field) and the geometry (WYSIWYG editor).
+All features can be deleted at once with a dedicated button, while individual
+features must be deleted in the edit popup window.
+
+In this application, background layers are tile-based MapQuest and OSM sources.
+On the other hand, foreground layers are (hand-selected) WFS resources.
+
+The application provides an "intersection mode", which can be switched ON/OFF
+via a checkbox at the top-right corner.
+When this mode is ON, data from foreground / WFS layers is restricted to areas
+which intersects with user features.
+This is achieved through an OGC ``Intersects`` filter within the WFS
+``GetFeature`` request.
+This data filtering could have been done at the client side, using OpenLayers
+capabilities. However, it would have incurred lots of data transfer and put a
+strain on the web browser memory usage.
+
+##### Known issues
+
+Having user features which geographically overlap breaks WFS requests made
+while "intersection mode" is active.  
+This is due to how the WFS ``GetFeature`` query + ``Intersects`` filter is constructed
+(``MultiPolygon`` made of the combination of all ``Polygon`` features).
 
 ### _Software libraries selected_