Doc should mention Adjust.onResume() prerequisite for event tracking

[AnwarShahriar]

My team was implementing Adjust for android by following the README.md and we avoided session tracking setup as we only wanted to track revenue event only.

Later we realized by trial and error that without setting up session tracking (Adjust.onResume() and Adjust.onPause() with activity lifecycle), the event tracking doesn't work at all.

The Github README.md event tracking section doesn't mension that we must implement session tracking to make it work. Updating the doc would be super helpful, let me know if I should open up a PR to do that or we're doing something entirely wrong here.