Updates README.md for Labs section.

Adds and fixes information for IRC, mailing lists. Adds information to contact
the training-infra sub-team leads on IRC or email.

Co-Authored-By: Shilla Saebi <shilla.saebi@gmail.com>
Change-Id: Id0ae28da8a2d65074d11811093fb5cc3d9cbe076
Implements: blueprint openstack-training-labs