Instruct GH to collapse certain file diffs by default

Since these are generated, viewing their diff is not of much value when viewing a commit or PR.

See https://github.com/github/linguist#using-gitattributes

.gitattributes

@@ -0,0 +1,7 @@
+**/zz_generated.*.go linguist-generated=true
+BUILD.bazel linguist-generated=true
+bindata.go linguist-generated=true
+/docs/cli/** linguist-generated=true
+/protokube/pkg/gossip/mesh/mesh.pb.go linguist-generated=true
+/protokube/pkg/gossip/mesh/mesh.proto linguist-generated=true
+/vendor/** linguist-generated=true