Instruction to install PowerShellGet without Install-Module

[leonyu]

Summary of the new feature / enhancement

Alpine Linux now has a powershell package but it does not come with Install-Module, so there is no way to install PowerShellGet using existing instructions.

Could we document instructions on how to install PowerShellGet without a prior installation?

Proposed technical implementation details (optional)

E.g.

• Identify library path of PowerShell installation
• Drop binary/source file in library path