[Annoucment] Building open source projects with FlubuCore

Simple example of a build script with FlubuCore for a .NET Core open source project was added. Build script can also be used as a template.

It covers most common build steps that are needed in an open source project: Fetching build version from a file or using GitVersion, updating projects version, building projects in the solution, running tests, packing and publishing a nuget package. It also covers how to run build script in Appveyor and Travis CI.