Add .gitattributes for correct line-breaks by checkout

[Tobias-B-Besemer]

I will suggest you to add a file ".gitattributes" for correct line-breaks by checkout to your repo.

We at https://github.com/bleachbit/bleachbit started to use a file ".gitattributes" to have correct line-breaks by checkout no matter if you checkout with Windows (CRLF), or with a Unix-like system (LF). This helps us e.g. by linting and solves some other issues that came up in the past... For the beginning I created a simple and basic ".gitattributes" file. In this file is e.g. defined, that NSI & NSH files from NSIS have ever a CRLF as line-break, because NSIS is a Windows installer and therefor Windows line-breaks make IMHO much sense... After introducing ".gitattributes" we got in some trouble with NsisMultiUser.nsh and UAC.nsh. I guess the problems come from because the files have no CRLF... (?) And so I report now this problem upstream (by you) and want suggest you to use a ".gitattributes" for checkout in future, too. The basic ".gitattributes" I have created is this PR. You can easily edit and improve it, or just use it as it is... ...if you use I will be really happy and thankful!