gstreamer: add caveat about other formulae needed (#20021)

Formula/gstreamer.rb

@@ -54,6 +54,14 @@ class Gstreamer < Formula
     system "make", "install"
   end
 
+  def caveats; <<~EOS
+    Consider also installing gst-plugins-base and gst-plugins-good.
+
+    The gst-plugins-* packages contain gstreamer-video-1.0, gstreamer-audio-1.0,
+    and other components needed by most gstreamer applications.
+    EOS
+  end
+
   test do
     system bin/"gst-inspect-1.0"
   end