Document `PHANPY_IMG_ALT_API_URL`

README.md

@@ -179,6 +179,9 @@ Available variables:
   - May specify a self-hosted Lingva instance, powered by either [lingva-translate](https://github.com/thedaviddelta/lingva-translate) or [lingva-api](https://github.com/cheeaun/lingva-api)
   - List of fallback instances hard-coded in `/.env`
   - [↗️ List of lingva-translate instances](https://github.com/thedaviddelta/lingva-translate?tab=readme-ov-file#instances)
+- `PHANPY_IMG_ALT_API_URL` (optional, no defaults):
+  - API endpoint for self-hosted instance of [img-alt-api](https://github.com/cheeaun/img-alt-api).
+  - If provided, a setting will appear for users to enable the image description generator in the composer. Disabled by default.
 - `PHANPY_GIPHY_API_KEY` (optional, no defaults):
   - API key for [GIPHY](https://developers.giphy.com/). See [API docs](https://developers.giphy.com/docs/api/).
   - If provided, a setting will appear for users to enable the GIF picker in the composer. Disabled by default.