add hounslow_gov_uk source

doc/source/hounslow_gov_uk.md

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+# London Borough of Hounslow
+
+Support for schedules provided by [London Borough of Hounslow](https://hounslow.gov.uk), serving London Borough of Hounslow, UK.
+
+## Configuration via configuration.yaml
+
+```yaml
+waste_collection_schedule:
+    sources:
+    - name: hounslow_gov_uk
+      args:
+        uprn: "UPRN"
+        
+```
+
+### Configuration Variables
+
+**uprn**  
+*(String | Integer) (required)*
+
+
+## Example
+
+```yaml
+waste_collection_schedule:
+    sources:
+    - name: hounslow_gov_uk
+      args:
+        uprn: "100021552942"
+        
+```
+
+## How to get the source argument
+
+### Using findmyaddress.co.uk
+
+An easy way to discover your Unique Property Reference Number (UPRN) is by going to <https://www.findmyaddress.co.uk/> and entering in your address details.
+
+### Using browsers developer tools
+
+- Go to <https://www.hounslow.gov.uk/homepage/86/recycling_and_waste_collection_day_finder#collectionday>
+- Enter your postcode and click "Find address"
+- Right on the `Pick an address` dropdown and select `Inspect` or `Inspect Element`
+- Expand the `select` element and find the `option` element with the you address
+- The first number in the `value` attribute is your UPRN