Add a minimal SimpleJIT example program.

This minimally demonstrates usage of the API, and serves as a very small
testcase to test that the basic JIT mechanisms are working.

lib/simplejit/README.md

@@ -2,3 +2,7 @@ This crate provides a simple JIT library that uses
 [Cranelift](https://crates.io/crates/cranelift).
 
 This crate is extremely experimental.
+
+See the [example program] for a brief overview of how to use this.
+
+[example program]: https://github.com/CraneStation/cranelift/tree/master/lib/simplejit/examples/simplejit-minimal.rs