docs: add documentation for how to contribute
basically something added to `README.md` in a "Contribute" section like...

if you wanna get started just fork the lad repo

then `cd lad` and `yarn install`, then you will need to `cd template/` and `yarn install`.  so there’s two places you have to install dependencies in

after that, just run `yarn start` from the `cd template/` folder and ur good to go

if you get an error about sharp package, then you need to install it, so see https://github.com/ladjs/lad/issues/272