SIL.FwBuildTasks: make the feature/nuget branch the main development branch

[ermshiperete]

• merge master into feature/nuget again
• announce that the merge of the feature/nuget branch will be made the main development branch
• create a git tag on master (pre-nuget ?) before doing the merge
• merge feature/nuget into master. Doing this requires adjusting the path to the CHANGELOG.md file in the .csproj as well as adjusting GitVersion.yml (removing settings for feature/nuget branch).
• create a release of the nuget packages, and use these released nuget packages in the other projects.

[tombogle]

This must be on Gerrit, too. There are no such branches visible on github.