Link to pyosmium-up-to-date (#402)

PLANET.md

@@ -21,6 +21,11 @@ First decide where to get the `planet.osm.pbf` file:
   osmium apply-changes daylight.osm.pbf admin.osc.bz2 <buildings.osc.bz2, ...> -o everything.osm.pbf
   osmium renumber everything.osm.pbf -o planet.osm.pbf
   ```
+- If you would like to update your `planet.osm.pbf` file to the latest changes from OpenStreetMap, you can use [pyosmium-up-to-date](https://docs.osmcode.org/pyosmium/latest/tools_uptodate.html):
+
+  ```bash
+  pyosmium-up-to-date --size 20000 -v planet.osm.pbf
+  ```
 
   NOTE: you need at least `admin.osc.bz2` for the `boundary` layer to show. This takes about 2.5 hours and needs as much
   RAM as the `planet.osm.pbf` size.