* Add instructions for multi-os linting
Added a matrix to show how to do multi-os lint builds. This is specifically needed if you have files like `_darwin.go` or `_windows.go`, etc. If you don't lint on that specific OS, then those files will NOT be linted.
I also added a note about `.gitattributes` needing to have the line endings correctly checked out, otherwise linting will fail for formatting as well.
