Clarify class library framework and version requirements for creating new script aliases

I don't know if there are ways of making things work with alternative Class Library project templates, but I tried .NET Standard and .NET Core and .NET Framework v4.5.2 (when part of a NuGet package) without any success. Ultimately, I had to specify my .NET Framework project to target v4.5[.0] to get it to work when pulling the package from NuGet.

If this is a solid requirement for Cake aliases, it might be worth a note when [instructing users to create a new class library](http://cakebuild.net/docs/fundamentals/aliases#creating-an-alias).

(Note: That .NET Framework v4.5.2 issue may have been my own working. I didn't investigate it further once I had v4.5 working.)