Add docs for gentype setup with commonjs

[alarbada]

I've been bitten by this same issue:
https://forum.rescript-lang.org/t/gentype-curry-js-error-when-exporting-a-rescript-function-with-more-than-1-parameter/2265

Adding the following config:

// bsconfig.json
"gentypeconfig": {
  ...,
  "module": "commonjs", // undocumented option
  ... etc
**},**

Solves it, but only thanks to searleser97 in the forum. If it wasn't for him I would have spent quite some time debugging this issue.

This option should be documented.