Consider adding a section for risk assessment to the RFC template

Over in #1, @ggreif [highlighted](https://github.com/bytecodealliance/rfcs/pull/1#discussion_r527914943) that the RFC template doesn't include a section on risk assessment:

> Should a proposal address some pain point, at the cost of potentially introducing other problems, one needs to elaborate the latter, and weigh in on their severity. One could argue that this belongs into the discussion of the proposal, but that thread is not a document.

Locate the RFC template referenced by this issue and read the discussion linked from #1 for the intended risk-assessment concern. Add a section that asks proposals to describe potential problems and weigh their severity, then verify the template presents this guidance clearly.

Written by the indexing model from the issue text.