I think it should be completely okay to "clutter" an interface with signatures, because you are definitely not supposed to have many methods in an interface, see the ISP. Plus, even if you don't change them often, they should still be easy to read too, especially in the Github editor.
I didn't make this clear in my post, sorry about that.
Talking about interfaces, I mean the implementations thereof, not that actual interface. The signature in the interface is something important, whereas the implementation I couldn't care less about.