Add Ruby to Rust example

ruby-to-rust/Makefile

@@ -0,0 +1,9 @@
+all: target/libdouble_input.so
+	ruby src/main.rb
+
+target/libdouble_input.so: src/lib.rs Cargo.toml
+	cargo build
+	(cd target && ln -nsf debug/libdouble_input-* libdouble_input.so)
+
+clean:
+	rm -rf target