Update the README of the text classification example
# What does this PR do?

This PR updates the main README of the examples folder and the one in the text classification example to take into account the recent changes in the scripts. In particular, I re-ran the command shown for all tasks with/without FP16 to make a clean table of results.

I moved all stuff about distributed training/TPUs in the general README of the examples as all example scripts now use Trainer, so have this working out of the box.
