[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.

Review the FlubuCore build-script example described in the issue, including its versioning, build, test, packaging, publishing, Appveyor, and Travis CI steps. The issue does not name files or define a requested change, so the expected completion criteria need to be clarified before work starts.

Written by the indexing model from the issue text.