message: Update HACKING.md for gsed
diff: @@ -9,6 +9,8 @@ work on CosmJS, i.e. modify it. It is not intended for users of CosmJS.
  - Node.js 12+, Docker and yarn
  - `sha256sum`, which you
  [can get on macOS as well](https://unix.stackexchange.com/questions/426837/no-sha256sum-in-macos)
  +- `gsed`, which you
  + [can get on macOS as well](https://formulae.brew.sh/formula/gnu-sed)
  ## Checking out code
  