add definition sources

[wdduncan]

The sources of definitions are not provided. This helpful for determining the reasons why a term is defined the way it is.

Two popular ways to do add definition sources:

1. Add an annotation to the definition annotation. For example, here is a screenshot of envo's definition of soil:

Second level annotations are added by clicking the "@" symbol.

2. Use a single definition source annotation. For example, here an example CIDO's definition and definition source for the call adverse event:

I was trained early in career using approach (2), but (1) has grown on me over time.