Document how to customize BETYdb

[dlebauer]

Can start by copy-pasting markdown from PecanProject/bety#447

[KristinaRiemer]

@dlebauer do you want to assign this to me?

[dlebauer]

@KristinaRiemer yes, please coordinate with @gsrohde about which branch to use for new content. I think he has been migrating from gitbook to bookdown but am not sure of the status