Added instructions for import in gradle files
Added instructions for import in gradle files

Included the instruction to import the amplitude library for projects
using gradle, the new build tool recommended by Google to create
android projects.

Signed-off-by: Joao Trindade <879066a13863d19049bd0f649ab4997f079729ad@gmail.com>

amplitude/Amplitude-Android,ifrade/Amplitude-Android,niqo01/Amplitude-Android