fixed CONTRIBUTING.md instructions

[ShiraiEd]

Adds

2. In the root directory run `git remote add upstream https://github.com/zaptech-dev/reeverb`
6. Run `cargo install trunk`
7. Run `rustup target add wasm32-unknown-unknown`
8. `cd` into `reeverb/crates/dashboard` and run `trunk build`

into CONTRIBUTING.md

I got troubled with the wasm32 part,
would be better to just add it into README.md instead?

Closes#23