docs(readme): add 'how it works' description

README.md

@@ -1,5 +1,14 @@
 # Replace Maps
 
-<img src="icons/icon.svg" alt="Blue icon with white border. Pin for current position on map" align="right" width="128" height="128" />
+<img src="icons/icon.svg" alt="Light blue icon with white border. Shape is a pin for the current position on a map." align="right" width="128" height="128" />
 
-Replace Maps iframes with OSM
+Replace Google Maps iFrames with OpenStreetMap
+
+## How it Works
+
+This browser extension intercepts web requests from frames in a page.
+If the request URL matches the syntax to a Google Maps map, the response will be replaced.
+The search parameters of the request are decoded and converted to compatible syntax.
+As a result, the request response is an extension page that contains a [Leaflet](https://leafletjs.com/) & [OSM](https://www.openstreetmap.org/) map.
+
+You can turn the extension off for every hostname by using the browser action button or by using the settings page.