[Figures]: update figures README with theme and cancer group info

[jharenza]

What module should be updated and why?

figures README needs to be updated to include cancer_group info/hex codes now that #1142 is merged and a unified publication theme per #761

What changes need to be made? Please provide enough detail for another participant to make the update.

Add information about theme and color palette for cancer_group as well as palette for other one-off panel plots which can be colored by levels (eg #1152 used the "jco" journal theme).

What input data should be used? Which data were used in the version being updated?

NA, README update

When do you expect the revised analysis will be completed?

next week

Who will complete this?

@jharenza

[jaclyn-taroni]

Addressed by #1172 and if there are items remaining, I think they are subsumed by #1176 at this point.