Adds Flax BERT finetuning example on GLUE
# What does this PR do?

Adds Flax BERT finetuning example which finetunes on one of the GLUE tasks.

I evaluated all tasks 5 times and added the average runs, the best run. and stdev in a table in the README. I used the seed of the best run as the default.

I also ran all experiments on three devices: 8 Cloud TPU-v3, 1 Cloud TPU-v3, 1 P100 GPU. I compared the runtimes and put them in another table in the README.

This PR was discussed over Slack with @patrickvonplaten and @sgugger .

## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [X] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [X] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case.
- [ ] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/master/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/master/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?