Stop recommending the use of Make.protobuf in README.

Make.protobuf was from the pre-go-tool world where Makefiles
were the recommended way to build Go programs. That's obsolete now,
and it doesn't improve the use of Makefiles if folk still use them.

Fixes #4.
diff --git a/README b/README
index 73a0461..e8cae71 100644
--- a/README
+++ b/README
@@ -112,19 +112,10 @@
-To build a package from test.proto and some other Go files, write a
-Makefile like this:
+To build a package from test.proto, write a Makefile like this:
-	include $(GOROOT)/src/Make.$(GOARCH)
-	TARG=path/to/example
-		test.pb.go\
-		other.go
-	include $(GOROOT)/src/Make.pkg
-	include $(GOROOT)/src/pkg/
+	test.pb.go: test.proto
+		protoc --go_out=. $<
 To create and play with a Test object from the example package,