add minimal documentation for the SQLite usage

docs/customize/SQLite.md

@@ -0,0 +1,55 @@
+A Nominatim database can be converted into an SQLite database and used as
+a read-only source for geocoding queries. This sections describes how to
+create and use an SQLite database.
+
+!!! danger
+    This feature is in an experimental state at the moment. Use at your own
+    risk.
+
+## Installing prerequisites
+
+To use a SQLite database, you need to install:
+
+* SQLite (>= 3.30)
+* Spatialite (> 5.0.0)
+
+On Ubuntu/Debian, you can run:
+
+    sudo apt install sqlite3 libsqlite3-mod-spatialite libspatialite7
+
+## Creating a new SQLite database
+
+Nominatim cannot import directly into SQLite database. Instead you have to
+first create a geocoding database in PostgreSQL by running a
+[regular Nominatim import](../admin/Import.md).
+
+Once this is done, the database can be converted to SQLite with
+
+    nominatim convert -o mydb.sqlite
+
+This will create a database where all geocoding functions are available.
+Depending on what functions you need, the database can be made smaller:
+
+* `--without-reverse` omits indexes only needed for reverse geocoding
+* `--without-search` omit tables and indexes used for forward search
+* `--without-details` leaves out extra information only available in the
+  details API
+
+## Using an SQLite database
+
+Once you have created the database, you can use it by simply pointing the
+database DSN to the SQLite file:
+
+    NOMINATIM_DATABASE_DSN=sqlite:dbname=mydb.sqlite
+
+Please note that SQLite support is only available for the Python frontend. To
+use the test server with an SQLite database, you therefore need to switch
+the frontend engine:
+
+    nominatim serve --engine falcon
+
+You need to install falcon or starlette for this, depending on which engine
+you choose.
+
+The CLI query commands and the library interface already use the new Python
+frontend and therefore work right out of the box.

docs/mkdocs.yml

@@ -40,6 +40,7 @@ nav:
         - 'Special Phrases': 'customize/Special-Phrases.md'
         - 'External data: US housenumbers from TIGER': 'customize/Tiger.md'
         - 'External data: Postcodes': 'customize/Postcodes.md'
+        - 'Conversion to SQLite': 'customize/SQLite.md'
     - 'Library Guide':
         - 'Getting Started': 'library/Getting-Started.md'
         - 'Nominatim API class': 'library/NominatimAPI.md'