Add .gitattributes file

This adds the .gitattributes file to the tree. I’ve added two
attributes: linguist-generated, and linguist-documentation.

GitHub’s Linguist will follow this when calculating our language stats
as well as in other cases like commit diffs.

Any additional attributes or other generated files are welcome to be added!

.gitattributes

@@ -0,0 +1,13 @@
+**/deps.nix linguist-generated
+**/node-packages.nix linguist-generated
+
+pkgs/applications/editors/emacs-modes/*-generated.nix linguist-generated
+pkgs/development/r-modules/*-packages.nix linguist-generated
+pkgs/development/haskell-modules/hackage-packages.nix linguist-generated
+pkgs/development/beam-modules/hex-packages.nix linguist-generated
+
+doc/** linguist-documentation
+doc/default.nix linguist-documentation=false
+
+nixos/doc/** linguist-documentation
+nixos/doc/default.nix linguist-documentation=false