Document the importSource key in the VDOM spec

[rmorshea]

This is not documented yet, and it needs to be. The importSource key comes from this data structure: https://github.com/idom-team/idom/blob/230794350d6a80457f15bf3d5e47f2794bbaef5c/idom/core/vdom.py#L10-L12

[rmorshea]

This is partially resolved by: #308

We should still explain what the source and fallback are.