Enabled Discussions forum in the repo

[hickeng]

I've enabled this forum because we've got a mix of people with and without access to internal messaging and not everyone is on the Alumni slack instance.

Inevitably this will become yet another messaging mechanism in which to check for answers, but it at least allows for some more structure for discussion specifically related to the use of the spreadsheet and data collection for filing prep.

I've created some preliminary categories, but without use it's hard to tell if they align - if you're not sure where to start a discussion, put it in General. I expect most discussion to be in Getting Started->Support for the near future.