automake: Don't whine about not having README

Index: Makefile.am
--- Makefile.am.orig
+++ Makefile.am
@@ -9,3 +9,4 @@ EXTRA_DIST = TODO acinclude.m4 autogen.sh fluidsynth.p
 DISTCLEANFILES = fluidsynth.pc
 pkgconfigdir = $(libdir)/pkgconfig
 pkgconfig_DATA = fluidsynth.pc
+README: README.md