add install guide for signed msix releases

INSTALL.md

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+# Installing Wino Mail (Fork Release)
+
+This fork is currently distributed as a signed `.msix` package using a self-signed certificate.
+
+Because of that, Windows will **not** trust the package by default until you install the included certificate.
+
+## Files you need
+
+From the release, download:
+
+- `Wino.Mail.WinUI_2.0.8.0_x64.msix`
+- the included certificate file (`.cer`)
+
+## Step 1: Install the certificate
+
+1. Double-click the `.cer` file.
+2. Click **Install Certificate**.
+3. Choose **Current User**.
+4. Select **Place all certificates in the following store**.
+5. Choose **Trusted People**.
+6. Finish the wizard.
+
+If Windows still refuses to install the app afterward, also install the same certificate into:
+
+- **Trusted Root Certification Authorities**
+
+## Step 2: Install the app package
+
+### Option A — Try normal install first
+
+Double-click the `.msix` package and follow the App Installer prompts.
+
+### Option B — Recommended fallback
+
+If App Installer shows a certificate trust error, open **PowerShell as Administrator** and run:
+
+```powershell
+Add-AppxPackage "C:\path\to\Wino.Mail.WinUI_2.0.8.0_x64.msix"
+```
+
+Replace the path with the actual location of the downloaded `.msix` file.
+
+## Verifying the install
+
+To confirm the app is installed, run:
+
+```powershell
+Get-AppxPackage | Where-Object Name -like "*Wino*"
+```
+
+You can then launch **Wino Mail** from the Start menu.
+
+## Uninstalling
+
+To uninstall the forked package:
+
+```powershell
+Get-AppxPackage MajjoDuran.WinoMail | Remove-AppxPackage
+```
+
+## Notes
+
+- This fork currently uses a self-signed certificate for distribution.
+- That means users must trust the included certificate before Windows will install the package cleanly.
+- The `.cer` file is safe to share publicly.
+- The private signing certificate (`.pfx`) must **never** be shared.