Allow postingrules element and children to the sendDocument call for a customer

[eelkevdbos]

Feature
Adds the postingrules to the sendDocument call for a customer by

• adding the appropriate models (e.g. CustomerPostingRule + Line)
• adding getters and setters on the Customer model
• adding additions to the CustomerDocument class to render them

Bonus

• Adds some development instructions

[rojtjo]

Thanks!

Released as 3.9.0 🚀