Add restriction to Contributing Guide

[RichardLitt]

I find it really tough to review submissions from people who work at, or who have made their product. I'd like to limit contributions to everything except for the "Remote DNA" section to people who haven't written the link they're submitting. I think it'll make reviews easier, and submissions more awesome.

We all love what we have made, and it's hard to tell a maker that you don't think their idea is awesome (whatever awesome means). I think it's important to have discretion, though, or else this list isn't useful.

Thoughts?