message: Add mention to avoid cl::opt for MLIR passes in the developer guide
diff: @@ -38,6 +38,12 @@ with the pass name. For example, the affine dialect provides a loop tiling pass
  that is registered on the command line as `-affine-tile`, and with a tile size
  option that can be set with `-affine-tile-size`.
  +We also avoid `cl::opt` to provide pass options in favor of the
  +[pass options](WritingAPass.md#instance-specific-pass-options) mechanism. This
  +allows for these options to be serialized in a pass pipeline description, as
  +well as passing different options to multiple instances of a pass in the same
  +pipeline.
  +
  ## Testing guidelines
  See here for the [testing guide](TestingGuide.md).
  