The solution is intended to be built with Visual Studio 2017; the .csproj should work with dotnet, however, the support for .NET 2.0/3.5 (the net20 / net35 targets) is not currently great in dotnet.
If you wish to build with the dotnet command line (but without net20 / net35 support), then run:
dotnet restore
dotnet build
or
dotnet restore
dotnet build -c Release
You may find it easier to use Visual Studio 2017, which can be downloaded here. I haven't tried it, but it should work with the "Community" edition, which is free.
To include the net20 / net35 targets: switch to the VS configuration.
If you're more of a "just give me the dll" person, it is available on nuget as protobuf-net, or:
install-package protobuf-net
in the Package Manager Console.