Add INSTALL file

[ycollet]

An INSTALL file describing how to compile the command line tool and the JUCE plugin should be added.

[julianstorer]

At the moment we've not yet released some of the source code that you'd need to build the command-line tool. When we do then of course we'll publish build instructions for it! :)

[ycollet]

Thank !
As soon as you release that, I will package soul for Fedora 32 / 33.

[julianstorer]

Thanks!