Updated the Design Proposal

I have added a new point to the proposal; the treatment of hitpoints (#14). I think this is a really cool way of handling damage, addressing many of the concerns raised against hit points over the years.

I have also changed the core mechanic. I feel like rolling under a number is much more realistic and provides much more reliable results over time than rolling above a target DC. The whole point of having attributes on a 3-18 scale is because it represents your relative strength vs a d20. If we are rolling d20s, I think it is the best system. However, as always, I am open to debate on this issue.