Add .gitattributes file.

Some ST resources need to have LF terminators to work as intented.
For example, letting Git handle line terminators automatically
causes problems with *.tmSnippet* files on Windows: LFs in the repo
get converted into CRLFs on Windows, and when you try to insert a
snippet on Windows, you get extraneous CR characters in your file.

A *.gitattributes* file allows us to specify line terminator
conversion based on file extension.