Add an "Alternatives Considered" section to the RFD template

[alilleybrinker]

Right now the RFD template doesn't capture alternatives that may have been considered in the design of a proposal, but not pursued. In today's QWG discussion, we agreed it would be useful to capture some of that information in RFDs, in part to "show our work" with the CVE Board, and enable them to understand some of the history and deliberation that goes into producing a proposal that will eventually go to the Board.