[Upstream] Add info about slide's Presenter notes
[Upstream] Add info about slide's Presenter notes

We agreed to add to presenter notes to make slides (re)usable by speakers
with different background knowledge. Add a Tip about how to enable Presenter
notes and Slide table.

Change-Id: I038c2780f1efe68956ccdcb7b9ad72381f1d077e

openstack/training-guides,openstack/training-guides