Documentation: Init a worker with env vars

I have a few env vars (RANDOM_SEED, THREADS) that I was setting in my pool init code and attempting to read in the workers. I realise now that I don't think that it is supported in node. I wonder if we could support starting the workers with some env vars set, similar to how spawn/exec allow setting env vars.

Something like:

```js
let pool = Pool(() => spawn(new Worker('./foo', {
env: {
RANDOM_SEED: 'kjhsdfjk',
THREADS: '8'
}
})));
```