Add documentation

Author: lbussell

documentation/install-netfx3.md

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+# How to install .NET Framework 3.5 on Windows Server Core container images
+
+## Windows Server Core 2022 and later
+
+Windows Server Core 2022 and later container images can install [.NET Framework 3.5](https://learn.microsoft.com/en-us/windows-hardware/manufacture/desktop/features-on-demand-non-language-fod?view=windows-11#-net-framework) as a [Feature on Demand](https://learn.microsoft.com/en-us/windows-hardware/manufacture/desktop/features-on-demand-v2--capabilities?view=windows-11).
+
+```Dockerfile
+# escape=`
+FROM mcr.microsoft.com/windows/servercore:ltsc2022-amd64
+
+RUN dism /Online /Add-Capability /CapabilityName:NetFx3
+```
+
+## Windows Server Core 2019 and earlier
+
+Windows Server Core 2019 and earlier can use Windows Update to install .NET Framework 3.5. See [Deploy .NET Framework 3.5 by using Deployment Image Servicing and Management (DISM)](https://learn.microsoft.com/windows-hardware/manufacture/desktop/deploy-net-framework-35-by-using-deployment-image-servicing-and-management--dism) for more info.
+
+```Dockerfile
+# escape=`
+FROM mcr.microsoft.com/windows/servercore:ltsc2019-amd64
+
+RUN `
+    # Enable Windows Update service
+    sc config wuauserv start= auto `
+    # Install .NET Framework 3.5
+    && dism /Online /Enable-Feature /FeatureName:NetFx3 /All `
+    # Disable Windows Update service
+    && sc config wuauserv start= disabled
+```