ENHANCEMENT: Add how to RETIRE a package to the docs

[bcurran3]

When retiring a Chocolatey Package, the following steps should be followed:

• Create a new version of the retired Chocolatey Package.
• Prepend [Retired] to the title of the package (e.g. <title="[Retired] Software Title" />
• Change the package version to 99.99.99.99. Alternatively 86.86.86.8.6 or 86.86.86.yearmonthday. (discussion time)!
• Update the package description: Why is the package being retired?
• Create a chocolateyInstall.ps1 script that is either blank or simply has a Write-Host with the reason for retirement. (Necessary or you'll get a pack error of no content.)
• Delete all other files except chocolateyInstall.ps1 (and possibly a RETIRED.txt that is displayed via chocolateyInstall.ps1).
• Remove the iconUrl.
• Remove bugTrackerUrl, docsUrl, licenseUrl, malingListUrl, projectSourceurl, releaseNotes, copyright, and requireLicenseAcceptance.
• change tags to retired (e.g. < tags>retired< /tags>)
• If the distribution point (website) for the program files has ceased to exist and the program is no longer available, unlist all versions from the package gallery, except the final deprecated version. If the program is still available for download you can leave working package versions listed.
• By following this process, any existing users who try to update the old package will automatically get the new package and (hopefully) be informed of the retirement.

Personally, I prefer "[RETIRED]" over "[Retired]" as the status will be more noticeable.

An example retired package of mine: https://chocolatey.org/packages/ellp/86.20181230