docs(readme): explain how to install previous versions
docs(readme): explain how to install previous versions

Closes #191

hitesh97/batarang,angular/batarang,SomeKittens/angularjs-batarang,hitesh97/batarang,erwinmombay/angularjs-batarang,jbnicolai/batarang,DrewML/batarang,erwinmombay/angularjs-batarang,DrewML/batarang,iegik/batarang,angular/batarang,SomeKittens/angularjs-batarang,seawenzhu/batarang,iegik/batarang,jbnicolai/batarang,jvilk/angularjs-batarang,jbnicolai/batarang,seawenzhu/batarang,erwinmombay/angularjs-batarang,jvilk/angularjs-batarang,vanessayuenn/batarang,seawenzhu/batarang,vanessayuenn/batarang,DrewML/batarang,iegik/batarang,vanessayuenn/batarang,angular/batarang,SomeKittens/angularjs-batarang,hitesh97/batarang,jvilk/angularjs-batarang