Encourage users to comment on why they up/down voted a requirement

[fxjordan]

After they initially voted, we could show an additional button on the details page, like "Comment on why you down vote this requirement"

Comments could then be annotated with the commentors vote (if they voted) so other users can better understand this reasoning.

Also, project developers and owners have better insights about the requirement if they can see reasons for why people up- or down-voted a requirement.