ffmpeg: add caveats to document AAC encoder switch

Formula/ffmpeg.rb

@@ -155,4 +155,16 @@ class Ffmpeg < Formula
     end
   end
 
+  def caveats; <<-EOS.undent
+    FFmpeg has been built without libfaac for licensing reasons.
+    To install with libfaac, you can:
+      brew reinstall ffmpeg --with-faac
+
+    You can also use the libvo-acenc or experimental FFmpeg encoder to
+    encode AAC audio:
+      -c:a libvo_aacenc
+    Or:
+      -c:a aac -strict -2
+    EOS
+  end unless build.with? "faac"
 end