Move readme figures to ./man/figures

[jeroen]

See https://docs.ropensci.org/rperseus/ and this comment from Hadley: ropensci-org/rotemplate#19 (comment)

You can also have a look where other packages store readme.md figures:

• https://github.com/ropensci/tidyhydat/
• https://github.com/ropensci/drake/