Add .NET Core to build instructions

[jokravit]

In the build instructions, you specify adding ".NET desktop development workload, as well as the .NET Framework 4.7 targeting pack" however, there is no mention of adding .NET Core (available here) which was required for me to successfully build. I believe checking that the ".NET Core cross-platform development" option in the installer would also be sufficient, but I did not test that.

[danbohus]

We have added the necessary documentation to the build instructions.