Rename why-grunt?.md to allow cloning in Windows

Let me preface this with "Har de har har Windows doesn't allow question marks in filenames oh Windows you're so goofy."

...but seriously, when I tried to clone down the docs repo to make a change about 0.4.3, I wasn't able to fully clone it down because Windows won't save `Why-grunt?.md` because a question mark is an invalid char in Windows filenames. I didn't see any explicit references to the file anywhere else in the repository, and this totally doesn't have to be accepted, but it does allow Windows users to get a full cloned version of the docs repo.