Add ALW Wolfenbüttel as source

doc/source/alw_wf_de.md

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+# ALW Wolfenbüttel
+
+Support for schedules provided by [ALW Wolfenbüttel](https://www.alw-wf.de//), Germany.
+
+## Configuration via configuration.yaml
+
+```yaml
+waste_collection_schedule:
+  sources:
+    - name: alw_wf_de
+      args:
+        ort: ORT
+        strasse: STRASSE
+```
+
+### Configuration Variables
+
+**ort**<br>
+*(string) (required)*
+
+**strasse**<br>
+*(string) (required)*
+
+## Example
+
+```yaml
+waste_collection_schedule:
+  sources:
+    - name: alw_wf_de
+      args:
+        ort: "Linden mit Okertalsiedlung"
+        strasse: "Am Buschkopf"
+```
+
+## How to get the source arguments
+
+1. Go to the calendar at https://www.alw-wf.de/index.php/abfallkalender.
+2. Select your location in the drop down menus.
+   - Notice: The page reloads after selecting `Ort`, so wait for that before selecting `Straße`.
+3. Copy the **exact** values from the 2 drop down menus as `ort` and `strasse` in the source configuration.
+   - In case you can only select the street `Alle Straßen`, then the `strasse` option does not matter (it is still required, just set it to something).