Add guidance for contributors
Add guidance for contributors

leftees/trade-tariff-backend,alphagov/trade-tariff-backend,alphagov/trade-tariff-backend,bitzesty/trade-tariff-backend,bitzesty/trade-tariff-backend,leftees/trade-tariff-backend,alphagov/trade-tariff-backend,bitzesty/trade-tariff-backend,leftees/trade-tariff-backend