Update README.md

[alerickson]

PR Summary

Updating instructions for building, testing, and importing a locally built version of PSResourceGet.

PR Context

PR Checklist

• [ ] PR has a meaningful title
  • Use the present tense and imperative mood when describing your changes
• [ ] Summarized changes
• [ ] Make sure all .h, .cpp, .cs, .ps1 and .psm1 files have the correct copyright header
• [ ] This PR is ready to merge and is not Work in Progress.
  • If the PR is work in progress, please add the prefix WIP: or [ WIP ] to the beginning of the title (the WIP bot will keep its status check at Pending while the prefix is present) and remove the prefix when the PR is ready.
• Breaking changes
  • [ ] None
  • OR
  • [ ] [Documentation needed]()
    • [ ] Issue filed:
• User-facing changes
  • [ ] Not Applicable
  • OR
  • [ ] [Documentation needed]()
    • [ ] Issue filed:
• Testing - New and feature
  • [ ] N/A or can only be tested interactively
  • OR
  • [ ] Make sure you've added a new test if existing tests do not effectively test the code changed
• Tooling
  • [ ] I have considered the user experience from a tooling perspective and don't believe tooling will be impacted.
  • OR
  • [ ] I have considered the user experience from a tooling perspective and enumerated concerns in the summary.