Add a note about autocrlf in the contributing book

[Burgov]

As discussed in symfony/symfony#2813

I'd make a PR, but I'm not sure which article is the best place for this

[pulzarraider]

I suggest Submitting a Patch. Here is chapter about initial GIT setting. CRLF problem on Windows platform should be a note.

[Burgov]

Ah cool, I missed that one. I'll try and post a PR somewhere today