Add a rough makefile for the documentation.

Just runs doxygen for us, an optionally latex.
The 'clean' target removes the doxygen output
directories.

doc/Makefile

+## GNU makefile for opusfile documentation.
+
+all: doxygen
+
+doxygen: Doxyfile ../include/opusfile.h
+	doxygen
+
+pdf: doxygen
+	make -C latex
+
+clean:
+	$(RM) -r html
+	$(RM) -r latex
+
+.PHONY: all clean doxygen