docs(readme): update readme (#7)

README.md

@@ -1,5 +1,82 @@
 # 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
+
+## Capabilities
+
+- replace Google Maps embeds with Leaflet map. Using API calls to:
+  - [Nominatim Search](https://nominatim.org/release-docs/develop/api/Search/)
+  - [OSM Tiles](https://www.openstreetmap.org/)
+  - [ArcGIS World Imagery Tiles](https://server.arcgisonline.com/ArcGIS/rest/services/World_Imagery/MapServer/)
+- disable extension for hostnames
+
+## Uncapabilities
+
+- replace maps requested from API from parent page
+- replace other map providers
+- send usage data and statistics
+
+## 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 response is an extension page that contains a [Leaflet](https://leafletjs.com/) + [OSM](https://www.openstreetmap.org/) + [ArgGIS](https://server.arcgisonline.com/ArcGIS/rest/services/World_Imagery/MapServer/) map.
+
+You can turn the extension off for every hostname by using the browser action button or by using the settings page.
+
+### Extension Flowchart
+
+```mermaid
+flowchart TD
+
+subgraph action [Browser Action]
+actionclick(Action Icon) -->|add / remove| storage[(Disabled Hostnames)]
+settings(Settings Page) -->|add / remove| storage
+end
+
+
+subgraph reqres [Request-Response Sytem]
+    req([Frame Request]) --> url{Matches\nGoogle Maps\nURL?}
+    url -->|no| nomatch([Continue Original Request])
+
+    url -->|yes| match{Hostname\nDisabled?}
+    storage -->|provide hostnames| match
+    match -->|no| res([Redirect to extension map page])
+    match -->|yes| nomatch
+
+end
+
+res -->|use params| params
+
+subgraph dec [Search-Param Decoding]
+    params(Search Params) -->|has q| q([readQ])
+    q -->|with title| pos[Marker/s]
+    params -->|has z| zoom[Zoom]
+
+    params -->|has pb| pb([readPB])
+    pb -->|has| minfo[Marker Info]
+    pb -->|has| marea[Map Area]
+
+    minfo -->|has\nsearch string| q
+    minfo -->|has\n0x...:0x...| cid[CID]
+    cid -.->|unknown usage| pos
+    minfo -->|has\nDMS coords| dms([parseDMS])
+    dms --> pos
+
+    pb -->|has| mtype[Map Type]
+
+    marea -->|has\ncoords| mcoords[Map Coords]
+    marea -->|has\naltitude| mzoom([getMapZoom])
+    mzoom --> zoom
+
+    pos --> mdata[(Map Data)]
+    mtype --> mdata
+    mcoords --> mdata
+    zoom --> mdata
+end
+
+mdata -->|use map data| mview([Load Leaflet Map])
+```