Documentation of svcutilcore

We need to have documentation for the Svcutilcore.

Per our discussion, here's the action items for this issue,

Create a .md file in https://github.com/dotnet/core/tree/master/samples for instruction and samples.
Open an issue in dotnet/announcements for the announcement of SvcutilCore. The issue would be locked for discussion.
Open an issue in dotnet/wcf for discussion and user feedback.