message: Document VM testing
diff: @@ -21,3 +21,20 @@ docker run --rm -v "$(pwd)":/code \
  confio/cosmwasm-opt:0.7.2 ./contracts/hackatom
  cp contracts/hackatom/contract.wasm lib/vm/testdata/contract_0.7.wasm
  ```
  +
  +## Testing
  +
  +By default, this repository is built and tested with the singlepass backend. This
  +requires running Rust nighty:
  +
  +```sh
  +cd lib/vm
  +cargo +nightly test
  +```
  +
  +To test with Rust stable, you need to switch to cranelift:
  +
  +```sh
  +cd lib/vm
  +cargo test --no-default-features --features default-cranelift
  +```
  