More robust README install instructions

[gbarter]

Purpose

Making README install instructions clear for all platforms whether installing fully from conda or installing NREL packages from source.

Type of change

• [ ] Bugfix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (non-backwards-compatible fix or feature)
• [ ] Code style update (formatting, renaming)
• [ ] Refactoring (no functional changes, no API changes)
• [x] Documentation update
• [ ] Maintenance update
• [ ] Other (please describe)

Testing

• [x] I have run existing tests which pass locally with my changes
• [ ] I have added new tests or examples that prove my fix is effective or that my feature works
• [ ] I have added necessary documentation