docs: add llvm to README.md

[thephez]

Issue being fixed or feature implemented

On Ubuntu, I had to install llvm to get yarn setup to work

What was done?

Added llvm to the list of packages to install

How Has This Been Tested?

Installed locally and then was able to run yarn setup

Breaking Changes

N/A

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated relevant unit/integration/functional/e2e tests
• I have added "!" to the title and described breaking changes in the corresponding section if my code contains any
• I have made corresponding changes to the documentation if needed

For repository code-owners and collaborators only

• I have assigned this pull request to a milestone