Update build instructions for CSharp wrappers
Building with CSharp Wrappers enabled may not be trivial for new developers. Update build instructions to include instructions to install swig, installation of .NET framework and SDK.
Additionally, it appears that the build specifically requires .NET framework 4.6.1 which is out of support now. Is this something that is governed by Unity or can we bump the required framework to the latest one or at least one that is under support. See this page for support info from Microsoft:
https://dotnet.microsoft.com/en-us/download/visual-studio-sdks