Adding usage instructions to example song file

It seems like it can't hurt to add usage instructions inside this
file, to avoid having to cross reference them from other places.

Author: jstrait

example_song.txt

@@ -1,4 +1,18 @@
-# An example song
+# An example Beats Drum Machine song!
+#
+# First, install Beats Drum Machine, using the instructions at
+# `https://beatsdrummachine.com/download/`.
+#
+# Next, download the Casio SA-20 sound files from the link at
+# `https://beatsdrummachine.com/download/#drumkit-casio` and add
+# them to a `sounds/` folder inside the same folder as this file.
+#
+# Finally, run the following command:
+#
+#   beats example_song.txt
+#
+# This should create an `example_song.wav` file in the current
+# folder.
 
 Song:
   Tempo: 120