civix entity - clarify civibuild needs

Two recent SE questions have made me think that readers are missing this
key set of installing CiviCRM with civibuild (instead of with the normal
installation steps). I've added clarification here which I hope will
prevent future users from stumbling on this step.

ref:

https://civicrm.stackexchange.com/questions/19294/how-to-create-entity-in-civicrm
https://civicrm.stackexchange.com/questions/19194/unable-to-locate-folder-xml-under-the-civicrm-root-directory