Update README Terminology to define and split out admin vs organizer terms

[allella]

• I have read Chapter's contributing guidelines.
• My pull request has a descriptive title (not a vague title like Update README.md).
• My pull request targets the master branch of Chapter.

Closes Nothing.

See my fork for a visual of the changes.

• The user roles were mixed into the terminology but not well defined.
• the use of the word "admin" in the chapter row was in conflict with the most recent confirmation of administrator vs organizer permissions.

[vkWeb]

I think we should confirm this with @QuincyLarson. If he agrees to the permissions we are giving to organizers and admin then I'll be happy to merge.

[allella]

Using @QuincyLarson 's "prior art" of moving things along if they sit unanswered, I'm merging this in since it's more clear than what we have and it's only a README change.

We're also having a conversation on #296 which depends on setting the user_role types, which should correspond to this terminology. We'll figure out if the terminology needs tweaking in the course of defining the user_role table.

[QuincyLarson]

@allella I'm a bit late to the party, but I just want to confirm - these are nice, succinct definitions.