Add PyInstaller spec and documentation

README.md

@@ -31,6 +31,7 @@ SeedPass now uses the `portalocker` library for cross-platform file locking. No
   - [Running the Application](#running-the-application)
   - [Managing Multiple Seeds](#managing-multiple-seeds)
     - [Additional Entry Types](#additional-entry-types)
+- [Building a standalone executable](#building-a-standalone-executable)
 - [Security Considerations](#security-considerations)
 - [Contributing](#contributing)
 - [License](#license)
@@ -503,6 +504,22 @@ python -m mutmut results
 
 Mutation testing is disabled in the GitHub workflow due to reliability issues and should be run on a desktop environment instead.
 
+## Building a standalone executable
+
+1. Run the vendoring script to bundle runtime dependencies:
+
+```bash
+scripts/vendor_dependencies.sh
+```
+
+2. Build the binary with PyInstaller:
+
+```bash
+pyinstaller SeedPass.spec
+```
+
+The standalone executable will appear in the `dist/` directory. This process works on Windows, macOS and Linux but you must build on each platform for a native binary.
+
 ## Security Considerations
 
 **Important:** The password you use to encrypt your parent seed is also required to decrypt the seed index data retrieved from Nostr. **It is imperative to remember this password** and be sure to use it with the same seed, as losing it means you won't be able to access your stored index. Secure your 12-word seed **and** your master password.