Add documentation about child-hook logic

[cnotv]

Description

Through the whole app is extensively used the child-hook as part of architecture used for the form, although it's not specified how this has to be used.

Specifically:

• Define concept
• Draw flow chart
• Components customization use case (via props)
• Exception use case: e.g. applyHooks is called with _beforeSaveHooks when clicking Edit as YAML (link)

https://github.com/rancher/dashboard/blob/master/shell/mixins/child-hook.js