Doc: Add note about AutoCRLF issue

[maxisam]

#264

[alerque]

If this is a necessary configuration on Windows shouldn't git-crypt itself be adding it the attributes?

[maxisam]

I think it is because autocrlf is default/recommend to auto in Windows. I believe it might work just fine with autocrlf=false.
I don't write c++ for 20 years, so I will not mess around the code. But I think this deserves a note in the Readme, since it caused me couple hours to figure this out.

[alerque]

Yes I understand how that setting would cause unexpected line endings, the surprising bit to me is that this hasn't been bumped into before by other Windows users since that's kind of the default (if not so much de facto) usage.

[maxisam]

Yeah, that makes two of us. But I don't use autocrl = true until 2 years ago. And this project doesn't have windows build until 0.7.0. So I guess not so many windows users here.