Add GH Actions badge to README

Author: TooTallNate

README.md

@@ -2,8 +2,7 @@
 
 ## Output [PCM audio][pcm] data to the speakers
 
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 A Writable stream instance that accepts [PCM audio][pcm] data and outputs it
 to the speakers. The output is backed by `mpg123`'s audio output modules, which