nav: add README (#26802)

* start docs

* Update README.md

selfdrive/navd/README.md

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+# navigation
+
+This directory contains two daemons, `navd` and `map_renderer`, which support navigation in the openpilot stack.
+
+### navd
+
+`navd` takes in a route through the `NavDestination` param and sends out two packets: `navRoute` and `navInstruction`. These packets contain the coordinates of the planned route and turn-by-turn instructions.
+
+### map renderer
+
+The map renderer listens for the `navRoute` and publishes a rendered map view over VisionIPC for the navigation model, which lives in `selfdrive/modeld/`. The rendered maps look like this:
+
+![](https://i.imgur.com/oZLfmwq.png)
+
+## development
+
+Currently, [mapbox](https://www.mapbox.com/) is used for navigation.
+
+* get an API token: https://docs.mapbox.com/help/glossary/access-token/
+* set an API token using the `MAPBOX_TOKEN` environment variable
+* routes/destinations are set through the `NavDestination` param
+  * use `set_destination.py` for debugging
+* edit the map: https://www.mapbox.com/contribute
+* mapbox API playground: https://docs.mapbox.com/playground/