[git] Add .gitattributes to remove unneeded files

[stof]

the bin folder should also be excluded, as it is a development tool

[Slamdunk]

I didn't add ./bin folder because often it's used to spread a distribution binary, like https://github.com/Behat/Behat/blob/master/bin/behat itself.

Although now there aren't distribution binaries, as a new contributor I do not feel safe by adding that folder, even suggested by a project member :fearful:

[stof]

Please add it. This package only has a development script (and there is no reason for us to ship a runtime CLI script in this package given its goal)

[Slamdunk]

Done :wink:

[Slamdunk]

Ping @stof

[stof]

thanks. and sorry for having forgotten it