Add a warning, explaining the correct use case for the scripts

[bekir-ozturk]

Addressing the issue here: dotnet/docs#18715

Some users perceive the install scripts as a replacement to the other installers (like msi): they expect to be able to access the installed runtime after powershell exits. This attempt fails since necessary environment variables will only work within the same powershell that installed the sdk (or scripts run from the same powershell).

Add a warning so that people who wants to install a runtime to run apps should use other installation options.

[bekir-ozturk]

Duplicate of #43, closing