message: Add a link to the rational on lack of const for IR units in the developer guide
diff: @@ -13,6 +13,7 @@ LLVM style guide:
  * Adopts [camelBack](https://llvm.org/docs/Proposals/VariableNames.html);
  * Except for IR units (Region, Block, and Operation), non-nullable output
  argument are passed by non-const reference in general.
  +* IR constructs are not designed for [const correctness](UsageOfConst.md).
  ## Pass name and other command line options
  