Add installation prerequisites to README.
Add installation prerequisites to README.

Document the dependency, as opposed to the non-idiomatic but well intentioned auto-install from #31
Cycling74/node-midi,justinlatimer/node-midi,drewish/node-midi,drewish/node-midi,Cycling74/node-midi,Cycling74/node-midi,szymonkaliski/node-midi,Cycling74/node-midi,julianduque/node-midi,justinlatimer/node-midi,szymonkaliski/node-midi,drewish/node-midi,julianduque/node-midi,brandly/node-midi,brandly/node-midi,brandly/node-midi,Cycling74/node-midi,julianduque/node-midi,drewish/node-midi,Cycling74/node-midi,szymonkaliski/node-midi,julianduque/node-midi,szymonkaliski/node-midi,drewish/node-midi,julianduque/node-midi,justinlatimer/node-midi,szymonkaliski/node-midi,brandly/node-midi,brandly/node-midi