Add documentation about using `Microsoft.Web.LibraryManager.Build`

[Piedone]

Is your feature request related to a problem? Please describe.

If I understand correctly, Microsoft.Web.LibraryManager.Build is the package to use when you want to integrate LibMan into your build (this is what our team needs, for people who just want to build the source, CI builds and for those working with IDEs different than Visual Studio). While it's great that this package exists, it's not mentioned anywhere in the Readme.

Describe the solution you'd like

A short mention of Microsoft.Web.LibraryManager.Build, and how to use it, under the Readme's Installation section. I don't think any more is needed than something along the lines of "install the latest version of package, add libman.json, enjoy". But the point here is that you can start with LibMan without Visual Studio or even the CLI if you know this.

Describe alternatives you've considered

Figuring out how to use it myself :).

Additional context

Nothing else.