Update doc to mention install of ffmpeg

I ran through this setup this morning and was able to get howdy working. I just wanted to update the doc to include a mention of the need to install ffmpeg. When I originally tried to run ffplay I got an error saying that I should try installing ffmpeg first. Hope it helps, thanks!
system76 · Nov 14, 2021 · 08fc67f · 08fc67f
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commit 08fc67f
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diff --git a/content/setup-face-recognition.md b/content/setup-face-recognition.md
@@ -98,6 +98,12 @@ ffplay /dev/INPUT
 
 Replace `INPUT` with results you had from previous command.
 
+Note: you may need to install `ffmpeg` before you are able to use the `ffplay` command. Install `ffmpeg` like this:
+
+```bash
+sudo apt install ffmpeg
+```
+
 #### Configuring Howdy to use webcam you want
 
 Once you find a webcam you want to use for Howdy, type in: