Add UPDATE docs

UPDATE.md

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+# Keep the vector tiles updated
+
+Once you have imported OpenMapTiles you can also keep it up to date by importing the latest OSM changes and
+regenerating the tables.
+
+## Import
+
+You can either keep the database up to date based on the daily OSM change feed
+or import specific change files.
+
+### Keep Database Updated
+
+You can use the new imposm3 feature to keep the database updated (thanks to the [work for @stirringhalo](https://github.com/openmaptiles/openmaptiles/pull/131)). This will automatically download
+the OSM change feed and import it into the database.
+After each run of you should also have a list of tiles that have updated.
+
+```
+docker-compose run update-osm
+```
+
+### Import Change File
+
+Given you have a file `changes.osc.gz` in your import folder. Once you ran the import command you should also have a list of tiles that have updated.
+
+```
+docker-compose run import-osm-diff
+```
+
+## Generate Changed Tiles
+
+After the import has finished **imposm3** will store a list of tiles in text format in the `diffdir`.
+Copy the as `tiles.txt` to the import folder.
+
+Now run the command to read the tilelist and write the vector tiles for it to a new MBTiles.
+
+```
+docker-compose run generate-changed-vectortiles
+```