Add optional dependencies section

README.md

@@ -130,6 +130,14 @@ data under `%LOCALAPPDATA%\WhispAssist`.
 
 Prefer the NSIS installer? Grab **`WhispAssist_<version>_x64-setup.exe`** from the same page.
 
+## Optional dependencies
+
+If you do not have these installed, WhispAssist will still work, but some features will be unavailable.
+
+- [yt-dlp](https://github.com/yt-dlp/yt-dlp/releases)
+- [ffmpeg](https://www.ffmpeg.org/download.html#build-windows)
+- [libpst](https://sourceforge.net/projects/ezwinports/files/libpst-0.6.63-w32-bin.zip/)
+
 ## Technology
 
 WhispAssist is a **Tauri 2** application: a small Rust core with a compiled **Svelte + TypeScript**