Document the differences between normal and design-time builds

I've glanced through this article:
https://github.com/dotnet/project-system/blob/master/docs/design-time-builds.md

But it doesn't seem to mention what the actual differences are between a design-time build and a regular command-line build. I'd like to be able to emulate a design-time build to an extent to be able to understand what is happening inside Visual Studio.

I can't remember the exact list, but I think there's a set of properties you need to set, such as:

```
DesignTimeBuild = true
BuildingInsideVisualStudio = true
BuildProjectReferences = false
SkipCompilerExecution = true
ProvideCommandLineArgs = true
```

Since the article above is the first place I'd go looking for this information it'd be nice to have it there. I'd send a PR but I'm not quite sure what is the exact set of properties and/or targets required.

It's OK if we can't fully emulate the design-time build, but it'd be nice to get to it as close as possible.

@davkean @panopticoncentral