Add annotations

[befriendabacterium]

Is your feature request related to a problem? Please describe.
Cannot find a way to add an annotation to PRISMA2020 diagram produced.

Describe the solution you'd like
Option to add annotation to specify extra detail e.g. on Web of Science databases searched. Something like the * and † annotations in the image below

Describe alternatives you've considered
Have tried looking at the DiagrammeR docs and stack discussions but there doesn't appear to be an easy way to add an annotation to the empty space in the diagramme, at least once it's produced. Might need to be done within-function as I appreciate DiagrammeR is a tricky package with unusual syntax to work with.

Additional context

P.S. Amazing package Chris and Neal!