docs: add ffmpeg requirement

Author: CyanVoxel

README.md

@@ -63,6 +63,8 @@ If you're interested in contributing to TagStudio, please take a look at the [co
 
 To download TagStudio, visit the [Releases](https://github.com/TagStudioDev/TagStudio/releases) section of the GitHub repository and download the latest release for your system under the "Assets" section. TagStudio is available for **Windows**, **macOS** _(Apple Silicon & Intel)_, and **Linux**. Windows and Linux builds are also available in portable versions if you want a more self-contained executable to move around.
 
+For video thumbnails and playback, you'll also need [FFmpeg](https://ffmpeg.org/download.html) installed on your system. 
+
 > [!IMPORTANT]
 > On macOS, you may be met with a message saying _""TagStudio" can't be opened because Apple cannot check it for malicious software."_ If you encounter this, then you'll need to go to the "Settings" app, navigate to "Privacy & Security", and scroll down to a section that says _""TagStudio" was blocked from use because it is not from an identified developer."_ Click the "Open Anyway" button to allow TagStudio to run. You should only have to do this once after downloading the application.