Update README.md re: unofficial Windows support

Unofficial Windows support blurb based on my testing of the ruby-lint gem on Windows.

Based on discussion from [Issue #131](https://github.com/YorickPeterse/ruby-lint/issues/131) and some more testing I've done, I feel fairly confident in telling the community that Windows isn't a total loss of a platform for ruby-lint. This simply documents that Windows has a fair chance of working, but Windows-specific problems likely won't be fixed by the maintainers. I think that's a good balance between trying to support every potential user, while not burdening the maintainers with platforms they prefer not to run themselves.