Clarify PowerShell policy

[mcandre]

Microsoft has changed the default PowerShell policy. The following requirements are now in effect:

• The execution policy must be opened up to at least RemoteSigned level, especially for typical desktop/laptop Windows envionments.
• Execution policy changes must be done in an elevated PowerShell context.
• PowerShell module installation must be done in an elevated PowerShell context.

Example:

1. Launch PowerShell in administrative mode.
2. Run Set-ExecutionPolicy -ExecutionPolicy RemoteSigned
3. Run Install-Module -Name posh-direnv

Reference:

https://docs.microsoft.com/en-us/powershell/module/microsoft.powershell.core/about/about_execution_policies?view=powershell-7.1

Could we update the README to reflect more of these relevant installation details?