Update Developer Documentation

[cdils]

Existing developer docs need to be updated prior to 2.0.0 release to reflect changes. Documentation should include:

• Theme requirements (i.e. PHP 7, npm, etc)
• Theme setup (i.e. npm install)
• Theme usage
• Theme upgrade path from < 2.0.0 (there is none, but we need to be explicit about it)
• Document 3rd party libraries and relevant configuration information
• I'm sure there are other things I'm forgetting.

@GaryJones I'll be looking for your help to document some of this. Okay, a lot of this. I'd kinda like to do it as a Github wiki, but don't think I can have a public wiki on a private repo.

Also dropping this here to remember to include info susy-breakpoint:
#107 (comment)

[GaryJones]

@cdils Are you planning on distributing the zip with the vendor/ directory already populated, or, since this is aimed at developers only, let them do composer install themselves?

[cdils]

@GaryJones No, they should run composer install.

[cdils]

I've created a new (public) repository to house UP documentation, setup, etc in the form of Wikis.

https://github.com/cdils/Utility-Pro-Documentation/

If you think this is a terrible idea, speak now. :)