Create guidance: how to develop and run a C# build task with source in the local repo

From https://github.com/dotnet/arcade/issues/96#issuecomment-375389291:

> [@tmat] ... The root problem here seems to be the inability to build and use a task "dynamically". But we do have a similar ability - the build can run a C# script. We even ship `csi` task that runs a specified C# script . There are some issues there that need to be fixed to make it smooth, but that can be done.

> Currently C# scripts do not support NuGet references (it's in the backlog, so maybe someday. However, that should be easy to work around. You can add those package references to the containing project and then access the binaries directly from the NuGet cache.

This repo should have a document about how we expect people to do this.

A side benefit is that we can have a consistent structure so moving the task to Arcade is painless.

No file or test is named. Start with the referenced comment in issue #96 and the shipped `csi` task described there; done means documenting a consistent way to develop and run a C# build task from local source, including the NuGet-reference workaround and considerations for moving it to Arcade.

Written by the indexing model from the issue text.