docs: document $ZPFX variable in README

[psprint]

Description

As @alichtman noted the $ZPFX variable isn't documented (https://github.com/zdharma-continuum/zinit/pull/547#issuecomment-1685376265). This PR adds information on the var to the readme.

Related Issue(s)

547

Motivation and Context

To make users understand what $ZPFX is and what configure'' and make'' ices are doing.

How Has This Been Tested?

With grip npm package.

Types of changes

• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] Documentation change
• [ ] New feature (non-breaking change which adds functionality)

Checklist:

• [x] All new and existing tests passed.
• [ ] I have added tests to cover my changes.
• [x] I have updated the documentation accordingly.

[psprint]

Thank you for the merge :+1: :-)

[github-actions[bot]]

:tada: This PR is included in version 3.12.0 :tada:

The release is available on GitHub release

Your semantic-release bot :package::rocket: