Doc: Updating tiles (#1340)

This tried to improve the documentation on Updates

UPDATE.md

@@ -1,13 +1,34 @@
-# Keep the vector tiles updated
+# Keeping 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
+You can either keep the database up to date based on the daily (or minutely) OSM change feed
 or import specific change files.
 
+### Choosing the Download Source
+
+While GeoFabrik currently provides extracts of basically all countries, they provide only daily updates. 
+If you need minutely updates you might want to try openstreetmap.fr, for example like this: `make download-osmfr area=africa/eritrea`, which configures minutely updates.
+
+### Preparations
+
+If you plan to keep data updated automatically, before importing any data, make sure to set 
+
+```
+DIFF_MODE=true
+```
+    
+in the `.env`
+
+Now download fresh data:
+
+``` 
+make download area=your-area-of-choice
+```
+
 ### Keep Database Updated
 
 You can use the new imposm3 feature to keep the database updated (thanks to the [work by @stirringhalo](https://github.com/openmaptiles/openmaptiles/pull/131)). This will automatically download
@@ -18,6 +39,28 @@ After each run you should also have a list of tiles that have updated.
 make update-osm
 ```
 
+#### Troubleshooting
+
+The output will be similar to this:
+
+``` 
+[info] Importing #4889572 including changes till ....... +0000 UTC (1m13s behind)
+``` 
+
+It might take some time to catch up with the latest changes, but the "time behind" should always decrease. If it doesn't, you need to download a new extract our don't have enough system resources to keep-up with the changes.
+
+Finally you will get an output like this - this indicates, that some 6 objects were changed:
+
+```
+[progress]     3s C:       0/s (0) N:       0/s (0) W:       0/s (6) R:      0/s (0)
+```
+
+The process will keep running forever and eventually print something like this - which just means that no changes were in the latest changeset:
+
+```
+[progress]     0s C:       0/s (0) N:       0/s (0) W:       0/s (0) R:      0/s (0)
+```
+
 ### 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.
@@ -31,11 +74,18 @@ make import-diff
 After the import has finished **imposm3** will store lists of tiles in text format in subfolders of the `diffdir`,
 named for the date(s) on which the import took place (`YYYYMMDD`).
 Copy and merge the files to `tiles.txt` in the import folder (`data`), either manually or with the following command, which also removes duplicate tiles so they are only generated once:  
+
 ```
 cd data && sort ./*/*.tiles | uniq > tiles.txt
 ```
 
-Now run the command to read the tilelist and write the vector tiles for it to a new MBTiles.
+After generating the tiles.txt you might and to delete the `*.tiles` files to not include them in the next run:
+
+```
+cd data && rm ./*/*.tiles
+```
+
+Finally run the command to read the tilelist and write the updated vector tiles in the existing MBtiles file.
 
 ```
 docker-compose run generate-changed-vectortiles