Be more clear in the genericscheme docs that custom interfaces are optional

Here: https://github.com/daostack/DAOstack-Hackers-Kit/blob/master/docs/gettingStarted/setupGenericScheme.md#how-to-get-your-generic-scheme-showing-up-in-alchemy

I think we should be explicit that developing custom interfaces in Alchemy is optional. Submitting the .json and the resulting default UI generated therefrom may be perfectly acceptable, and much easier.