[Drupal] README: Integrate permissions into README

[AnthonyAstige]

We should keep all our documentation in a single place, in the repository, with revision control. Foodtruck permissions seems like a useful document looking forward.

TODO

• Await Foodtruck Permissions to stabilize (good to edit in Google Docs while it's changing a lot)
• Merge into /drupal-project/README.md as replacing link hacked in at 6aad749

[AnthonyAstige]

@Lowell20 I'm assigning to you to check off the first item when the permission specs have stabilized. After that we can hand it to someone that wants to become more familiar with markdown (this is an easy task for a less experienced developer).