wash build fails with "wit-bindgen-wrpc": executable file not found in $PATH

```
main.go:1: running "wit-bindgen-wrpc": exec: "wit-bindgen-wrpc": executable file not found in $PATH
``` 
Readme should mention that `wit-bindgen-wrpc-cli` should be installed prior to build. 

```
cargo install wit-bindgen-wrpc-cli
```