Xiaomi-mi-scale

Added
- repository.json to make it a real add-on repo (fixes https://github.com/lolouk44/hassio-addons/issues/4)
Changed
- Now truly handles optional config entries(fixes https://github.com/lolouk44/hassio-addons/issues/3)
- MQTT Discovery set wtih retain flag (fixes https://github.com/lolouk44/hassio-addons/issues/2)
- README updated to use Xiaomi Mi Fit App to retrieve the MAC Address (fixes https://github.com/lolouk44/xiaomi_mi_scale/pull/25)

Added
- Docker Image so install is quicker (no local build required).

Added
- MQTT Discovery Support.

Added
- First release (version in line with non Add-On script for ease of maintenance).