Add issue template for missing model information (#1200)

Author: rytilahti

.github/ISSUE_TEMPLATE/missing-model.md

@@ -0,0 +1,26 @@
+---
+name: Missing model information for a supported device
+about: Inform about functioning device that prints out a warning about unsupported model
+title: ''
+labels: missing model
+assignees: ''
+
+---
+
+If you are receiving a warning indicating an unsupported model (`Found an unsupported model '<model>' for class '<class>'.`),
+this means that the implementation does not list your model as supported.
+
+If it is working fine for you nevertheless, feel free to open an issue or create a PR to add the model to the `_supported_models` ([example](https://github.com/rytilahti/python-miio/blob/72cd423433ad71918b5a8e55833a5b2eda9877a5/miio/integrations/vacuum/roborock/vacuum.py#L125-L153)) for that class.
+
+Before submitting, use the search to see if there is an existing issue for the device model, thanks!
+
+**Device information:**
+
+  - Name(s) of the device:
+  - Link:
+
+Use `miiocli device --ip <ip address> --token <token>`.
+
+  - Model: [e.g., lumi.gateway.v3]
+  - Hardware version:
+  - Firmware version: