update release template

Author: modelorona

.github/RELEASE_TEMPLATE.md

@@ -14,6 +14,26 @@ sudo snap install whodb
 ```
 [View on Snapcraft](https://snapcraft.io/whodb)
 
+### AppImage (Linux)
+
+Download the AppImage for your architecture from the assets below, make it executable, and run:
+
+```bash
+# For AMD64/x86_64
+chmod +x WhoDB-{{VERSION}}-amd64.AppImage
+./WhoDB-{{VERSION}}-amd64.AppImage
+
+# For ARM64/aarch64
+chmod +x WhoDB-{{VERSION}}-arm64.AppImage
+./WhoDB-{{VERSION}}-arm64.AppImage
+```
+
+All AppImages are signed with Sigstore. To verify:
+
+```bash
+cosign verify-blob --signature WhoDB-{{VERSION}}-amd64.AppImage.sig --certificate WhoDB-{{VERSION}}-amd64.AppImage.pem WhoDB-{{VERSION}}-amd64.AppImage
+```
+
 ### Docker
 ```bash
 docker pull clidey/whodb:{{VERSION}}
@@ -33,6 +53,7 @@ See assets below for platform-specific packages (DMG, MSIX, etc.).
 To upgrade from a previous version:
 - **Docker**: Pull the latest image and restart your container
 - **Snap**: Run `sudo snap refresh whodb`
+- **AppImage**: Download the new AppImage and replace the old one
 - **Desktop Apps**: Download and install the new version
 
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