Update the FAQ on reproducibility: PYTHONHASHSEED=0 before startup + TF on GPU issues

[ageron]

Summary

Python reads the PYTHONHASHSEED environment variable upon startup, so it's no use setting it from within the program.
It's also worth mentioning reproducibility issues when using the TensorFlow backend on a GPU. I just recommend running on the CPU if you need perfect reproducibility.

PR Overview

• [n] This PR requires new unit tests [y/n] (make sure tests are included)
• [n] This PR requires to update the documentation [y/n] (make sure the docs are up-to-date)
• [y] This PR is backwards compatible [y/n]
• [n] This PR changes the current API [y/n] (all API changes need to be approved by fchollet)

[fchollet]

LGTM, thanks!