📝 add documentation about helper scripts to install legacy .net sdks

Author: fluffynuts

README.md

@@ -27,3 +27,19 @@ tracker is no longer accepting new issues but is available at
 https://issues.apache.org/jira/browse/LOG4NET for access to 
 previous issues. 
 
+
+Developing
+==========
+
+log4net targets a wide array of .net platforms, including some
+which are out of support from Microsoft, making it difficult to
+install relevant SDKs and build for those targets. In particular,
+older Client Profile .NET Framework targets and dotnet core 1.1
+may be installed by using the bundled helper scripts:
+
+- [install-net-framework-sdk-3.5.ps1]()
+- [install-dotnet-core-sdk-1.1.ps1]()
+
+These scripts download the relevant installers from Microsoft servers,
+but you run them at your own risk.
+