Add .gitattributes to prevent .gitignore getting
inside release zips.

new git tags can be exported using:
git archive --prefix=plugin.video.nrk/ --output plugin.video.nrk-x.y.z.zip vx.y.z
Merge pull request #22 from torstehu/gitignore-export

Add .gitattributes to prevent .gitignore getting inside release zips.