Put install instructions of test tools into separate docs/ markdown file

docs/admin/Update.md

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+# Updating the Database
+
+There are many different ways to update your Nominatim database.
+The following section describes how to keep it up-to-date with Pyosmium.
+For a list of other methods see the output of `./utils/update.php --help`.
+
+!!! warning
+    If you have configured a flatnode file for the import, then you
+    need to keep this flatnode file around for updates as well.
+
+#### Installing the newest version of Pyosmium
+
+It is recommended to install Pyosmium via pip. Make sure to use python3.
+Run (as the same user who will later run the updates):
+
+```sh
+pip3 install --user osmium
+```
+
+Nominatim needs a tool called `pyosmium-get-updates` which comes with
+Pyosmium. You need to tell Nominatim where to find it. Add the
+following line to your `settings/local.php`:
+
+    @define('CONST_Pyosmium_Binary', '/home/user/.local/bin/pyosmium-get-changes');
+
+The path above is fine if you used the `--user` parameter with pip.
+Replace `user` with your user name.
+
+#### Setting up the update process
+
+Next the update needs to be initialised. By default Nominatim is configured
+to update using the global minutely diffs.
+
+If you want a different update source you will need to add some settings
+to `settings/local.php`. For example, to use the daily country extracts
+diffs for Ireland from Geofabrik add the following:
+
+    // base URL of the replication service
+    @define('CONST_Replication_Url', 'https://download.geofabrik.de/europe/ireland-and-northern-ireland-updates');
+    // How often upstream publishes diffs
+    @define('CONST_Replication_Update_Interval', '86400');
+    // How long to sleep if no update found yet
+    @define('CONST_Replication_Recheck_Interval', '900');
+
+To set up the update process now run the following command:
+
+    ./utils/update.php --init-updates
+
+It outputs the date where updates will start. Recheck that this date is
+what you expect.
+
+The `--init-updates` command needs to be rerun whenever the replication service
+is changed.
+
+#### Updating Nominatim
+
+The following command will keep your database constantly up to date:
+
+    ./utils/update.php --import-osmosis-all
+
+(Note that even though the old name "import-osmosis-all" has been kept for
+compatibility reasons, Osmosis is not required to run this - it uses pyosmium
+behind the scenes.)
+
+If you have imported multiple country extracts and want to keep them
+up-to-date, [Advanced installations section](Advanced-Installations.md) contains instructions 
+to set up and update multiple country extracts.