[Chore] Update README for libsodium dependency

Description

Problem: Quoting !1275 (closed):

I had a hard time getting this installed on my machine until I figured out that I had to install libsodium first. Then everything worked great. It would have saved me a couple hours of sleuthing if I had known I needed to do this up front. So I added this to the README.

Solution: Update the readme.

Related issue(s)

Resolves None

✅ 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.