README: added CI badge

README.md

 # Smart Home block-list firewall
 
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 Profile-based, multi-layer, block-list Smart Home firewall, based on NFTables & NFQueue.
 Minimal version of [smart-home-firewall](https://github.com/smart-home-network-security/smart-home-firewall),
 changed from an allow-list firewall to a block-list one.