Expand README's installation instructions

[mepc36]

I've never used JUCE before, so these instructions are blocking me right now:

Download and install [juce](https://juce.com/get-juce/download), including Projucer.
Build this plugin in visual studio after using projucer.

As I go about my installation project I will come back here and update with instructions for any other non-C++ devs like me out there.