📝 add information about helper scripts to install legacy .net/dotnet sdks to build instructions

Author: fluffynuts

BUILDING.md

@@ -18,6 +18,13 @@ Options:
             - on Win10+, this can only be installed via Add/Remove Windows Components
             - on other platforms, see https://dotnet.microsoft.com/download/dotnet-framework/net35-sp1
             - Building against .net 2/3.5, especially Client Profile, is not supported on Mono
+        - Log4Net supports some older, out-of-support .net SDKs, particularly
+          dotnet core 1.1 and .net framework client profiles for 3.5 and 4.0.
+          There are helper powershell scripts in the root of this
+          repository to assist with obtaining and installing these
+          SDKs from Microsoft servers. Please see:
+            - [install-dotnet-core-sdk-1.1.ps1](install-dotnet-core-sdk-1.1.ps1)
+            - [install-net-framework-sdk-3.5.ps1](install-net-framework-sdk-3.5.ps1)
     - Binaries can be built with a Visual Studio or Rider installation
     - Binaries, packages and a release zip can be built via commandline
         - Ensure that you have a reasonably modern NodeJS installed (at least version 8+)