Fixes #5081 - add instruction for install

[sdwheeler]

PR Summary

Fixes #5081 - add instruction for install
Fixes AB#1644934

PR Context

Select the type(s) of documents being changed.

Cmdlet reference & about_ topics

• Version 7 content
• Version 6 content
• Version 5.1 content

Conceptual articles

• Fundamental conceptual articles
• Script sample articles
• DSC articles
• Gallery articles
• JEA articles
• WMF articles
• SDK articles

PR Checklist

• I have read the contributors guide and followed the style and process guidelines
• PR has a meaningful title
• PR is targeted at the staging branch
• All relevant versions updated
• Includes content related to issues and PRs - see Closing issues using keywords.
• 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 and remove the prefix when the PR is ready.

[opbld30]

Docs Build status updates of commit 2ef0f1d:

✅ Validation status: passed

File	Status	Preview URL	Details
reference/5.1/PowershellGet/Save-Module.md	✅Succeeded	View (powershell-5.1)
reference/6/PowerShellGet/Save-Module.md	✅Succeeded	View (powershell-6)
reference/7/PowerShellGet/Save-Module.md	✅Succeeded	View (powershell-7)

For more details, please refer to the build report.

Note: If you changed an existing file name or deleted a file, broken links in other files to the deleted or renamed file are listed only in the full build report.

[davidsmatlak]

LGTM.