In notebook_template.ipynb, add a blurb for Googlers to inform viewers that the notebook is created by Googlers.

[ivanmkc]

Problem: Some feedback has been given that users may think the "community" notebooks are not authored by Google. In fact, "community" notebooks can be authored by anyone (Googlers and non-Googlers alike), but are just not maintained.

Although this is described in the repo's README, users might not read that. To mitigate this, we propose the creation of a short blurb in the notebook_template.ipynb that tells users that the notebook is Google-authored.