Added link to ffmpeg compositing explanation in movie background exam…

…ple.

movie_background/movie_background.md

@@ -13,9 +13,8 @@ To combine the transparent image frames and the background into a movie I'll use
 	$ cd ~/Desktop/movie
 	$ ~/Desktop/ChimeraX-1.8.app/Contents/bin/ffmpeg -loop 1 -i background.png -i chimovie_Umkk-%05d.png -filter_complex overlay=shortest=1 movie.mp4
 
-Here is where I got this ffmpeg compositing trick
+This ffmpeg compositing command is explained [here](https://stackoverflow.com/questions/10438713/overlay-animated-images-with-transparency-over-a-static-background-image-using-f).
 
-	[https://stackoverflow.com/questions/10438713/overlay-animated-images-with-transparency-over-a-static-background-image-using-f](https://stackoverflow.com/questions/10438713/overlay-animated-images-with-transparency-over-a-static-background-image-using-f)
 
 <video width="538" height="563" controls>
   <source src="movie.mp4" type="video/mp4">