[swift-evolution] [Meta] Updating the proposal template to prompt backward compatibility concerns

[Joe Groff]

With the end of Swift 3 drawing near, we'll need to start designing with an eye toward backward compatibility. To that end, I think we should update the "Impact on existing code" proposal template to start asking writers to consider backward compatibility concerns. Here's some draft verbage to that effect:

https://github.com/jckarter/swift-evolution/blob/4bdd1b875a8402ec6bbabb3f37151262f130af2b/0000-template.md#backward-compatibility

Any concerns, revisions, or objections?

-Joe