diff --git a/README.md b/README.md
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--- a/README.md
+++ b/README.md
@@ -2,6 +2,81 @@
 
 Conversion of chart data from different sources to create free nautical charts.
 
-:point_right: [Documentation](http://freenauticalchart.net/download/)
+:point_right: [Documentation](https://freenauticalchart.net/download/)
 
 ![example map](docs/img/chartimage.webp)
+
+> **Not for navigation.** The charts and data are for information, reference and
+> training only. See the [disclaimer](https://freenauticalchart.net/download/#important-information)
+> and the [legal notice](docs/legal.md).
+
+## What this project does
+
+This repository contains the scripts, styles and build files behind
+[freenauticalchart.net](https://freenauticalchart.net/). It takes open data for
+German and Dutch waters, converts it and publishes:
+
+- raster charts (`mbtiles`, `sqlitedb`) for AvNav, OpenCPN, OsmAnd and JOSM
+- S-57 vector charts (`zip`) for AvNav (Ocharts NG plugin) and OpenCPN
+- OsmAnd files (`obf`): depth data, light sectors and the marine rendering style
+- the [web chart](https://freenauticalchart.net/) and its [vector version](https://freenauticalchart.net/vector/)
+- tide curves, a tidal atlas and printable charts
+
+Ready-made files are on the [download page](https://freenauticalchart.net/download/),
+with instructions for each app.
+
+## Data sources
+
+- Germany: [BSH](https://www.bsh.de/) (Bundesamt für Seeschifffahrt und Hydrographie).
+  Parts of the BSH data are no longer freely available, so the German chart is now
+  based on older data; see the warning on the download page.
+- Netherlands: [Rijkswaterstaat](https://www.rijkswaterstaat.nl/) (ENC, buoys and beacons).
+
+The full list of sources and their licences is on the
+[download page](https://freenauticalchart.net/download/). Background on using BSH
+data (in German): [BSH.md](BSH.md) and [opendata.md](opendata.md).
+
+## Licence
+
+- Code: [GPL-3.0](LICENSE)
+- Charts and other downloadable files: [CC0](https://creativecommons.org/publicdomain/zero/1.0/)
+
+## Repository layout
+
+- `docs/` documentation site (MkDocs, English, German and Dutch)
+- `scripts/` Python tools for converting S-57, BSH and RWS data, tiles and light sectors
+- `qgis/` QGIS projects used to render the raster charts
+- `mapproxy/` MapProxy configuration for serving and seeding raster tiles
+- `web/` the web chart (Vite, Leaflet, MapLibre)
+- `vector/` the vector chart: tiles, styles and sprites (tippecanoe, pmtiles, MapLibre)
+- `osmand/` OsmAnd rendering style and map creation settings
+- `tides/` tidal atlas and tide curves (see [tides/README.md](tides/README.md))
+- `garmin/` Garmin map style
+- `icons/` chart symbols
+- `www/` output folder for the website
+
+## Building
+
+The full build downloads a lot of data and takes a long time. It is written for Linux.
+
+Tools used by the makefiles:
+
+- `make`, `wget`, `jq`, GNU `parallel`, `zipmerge` (libzip)
+- Python 3 with the packages in `scripts/requirements.txt`
+- GDAL (`ogr2ogr`) and SpatiaLite
+- QGIS Server and MapProxy (or Docker, see below)
+- Node.js with `npm` (web chart) and `pnpm` (vector chart)
+- Java (OsmAnd map creator)
+- MkDocs and its plugins (`cd docs && make install`)
+- for `vector/`: `make -C vector tools` builds tippecanoe, pmtiles and spreet
+  from source (needs Go, Rust and a C++ compiler)
+
+Main `make` targets:
+
+- `make build` full build: data, charts, tiles, vector charts and the website
+- `make docker`, `make docker-seed` run QGIS Server and MapProxy in Docker and seed the tile cache
+- `make qgis`, `make mapproxy`, `make seed` the same without Docker
+- `make web` build the web chart into `www/`
+- `make www` build the documentation into `www/download/`
+- `make serve` serve `www/` locally on port 8002
+- `cd docs && make serve` preview the documentation while editing