update code/morley/README.md for libsodium dependency

Description

I added a short paragraph to code/morley/README.md explaining that for cabal and stack builds, one must first install libsodium. I did this because I wasted a couple hours figuring out I needed this, and wanted to help others avoid that.

Related issue(s)

None

Resolves #

✅ Checklist for your Merge Request

Related changes (conditional)

• Tests (see short guidelines)

  • If I added new functionality, I added tests covering it.
  • If I fixed a bug, I added a regression test to prevent the bug from silently reappearing again.

• Documentation

  • I checked whether I should update the docs and did so if necessary:
    • Root README and other README.md files
    • Haddock
    • docs/
  • I updated changelog files of all affected packages released to Hackage if my changes are externally visible.

Stylistic guide (mandatory)

• My commits comply with the following policy.
• My code complies with the style guide.