chore: prepare repo for HACS listing

- Add hacs.json and MIT LICENSE required for HACS/default-store inclusion
- Fill required manifest.json keys (documentation, issue_tracker,
  codeowners) and reorder per hassfest's key-ordering rule
- Add validate.yml workflow running hassfest and HACS validation
- Update README install note now that hacs.json is checked in

README.md

@@ -60,7 +60,7 @@ This repo doesn't include the needed CA bundle. For an example of how to obtain
 
 ## Part 3: Add the integration in Home Assistant
 
-1. Copy `custom_components/localthings/` into your HA config's `custom_components/` directory. (Or install via a custom HACS repository: no `hacs.json` is checked in yet, so add this repo as a custom integration repository in HACS if you use it that way.)
+1. Copy `custom_components/localthings/` into your HA config's `custom_components/` directory. (Or add this repo as a custom repository in HACS — `Integration` category — and install it from there.)
 2. Restart HA.
 3. **Settings > Devices & Services > Add Integration > Local Things.**
 4. First device: paste the appliance's IP, plus the contents of the CA private and public key from Part 2.