updates language to be more open per #457

[dgreene1]

fixes #457 by using language that clarifies that we are open to refactoring

[HoldYourWaffle]

I'm just going for small iterative improvements here, perhaps something like this would be better?
(I know this is just "my writing preference", I just want the best for the project)

In situations where the code is very hard to extend, we may want to refactor or redesign certain aspects of the codebase. Such refactors should be proposed and agreed upon beforehand in the issue you created for your bugfix or feature proposal. While we strongly value enhancements to code quality, significant refactoring should usually be reserved for situations where the non-refactored version of code would be too difficult to work with.

Perhaps we should also mention something about the available time to review for maintainer(s)?