Enable ormolu only on certain projects

The documentation for ormolu-format-on-save says:

To enable it in specific files or directories,
use the local variables "mode" mechanism, e.g. in ".dir-locals.el" you
might use:

        (mode . ormolu-format-on-save)))

As ormolu works with Haskell sources, that “some-major-mode” has to be haskell-mode, so create a file named .dir-locals.el in the project root directory containing:

  (mode . ormolu-format-on-save)))

For now, I just have to manually format using SPC m f