Document best practices for creating Scores

[kirbysayshi]

It would be nice to have a "handbook" of how to construct and manipulate Scores, and the best practices therein.

One example is constructing all Scores to have a single terminal Gain node, even if it's not used. This allows a later modification to the Score to have a known starting point, and might also make composable modification functions possible. And if #3 is implemented, it means nearly no runtime cost either.