error: no such subcommand: `elf2sgxs`

**Problem**: The following command, as given in the `README.md`, fails

```console
cargo elf2sgxs --release
```

**Solution**: Install `elf2sgxs`:

```console
cargo install oasis-core-tools \
        --git https://github.com/oasisprotocol/oasis-core \
        --force \
        --locked
```

This could be added in the `README.md` or the oasis-core docs.
