Add to contributor guide instructions on hooking up state taxes to net income tree

* Add to `state_income_tax_before_refundable_credits`, `household_tax_before_refundable_credits`, and `household_refundable_tax_credits`
* Add `parameters/gov/states/nd/README.md` with `# North Dakota`
* Add to `modelled_policies.yaml`

Start by reading the definitions of state_income_tax_before_refundable_credits, household_tax_before_refundable_credits, and household_refundable_tax_credits, then inspect parameters/gov/states/nd/README.md and modelled_policies.yaml. Done means the state-tax hookup is reflected in all three named model entries, the North Dakota README exists with its requested heading, and modelled_policies.yaml is updated.

Written by the indexing model from the issue text.