Add box phar compilation, make compile command, and installation notes

[garettrobson]

I'm not sure what tools were intended to generate a Phar, but I added some support for box.

The box.json allows us to box compile; make compile just calls that.

Also added some notes to README.md to cover installation to /usr/local/bin, based on the latter steps in PHP-CS-Fixer - Global (Manual)

[martin-helmich]

Hey 👋 -- thanks for the contribution; I didn't know about Box before. At the moment, the actual release PHAR is built via some hacky shell scripting.

If it's alright with you, before merging this (and ending up having two different PHAR builds in the same repo), I'd prefer to play around with this a bit and see if we can maybe also replace the actual release build with a Box build. Stay tuned... ⏳

[garettrobson]

Absolutely understandable. I totally blanked the .build/package.sh file; for some reason I thought it was related to the docs 🤦

As it is I think you're far better suited than me for doing the fine tuning with box so I look forward to seeing what comes out the other end of this. I'll leave this PR up for reference but when you implement something just reject it.

Thank you for your time and effort :)