add more info about OAuth and WebClientID to README.md

[hyst3r1a]

My company spent a lot of time testing and troubleshooting this, so it seems like a good idea to add those lines for anyone who would be stuck at the same place. Kinda easy to skip this step, if one had never been using Google Cloud Console. Firebase key is also useful, and saves time determining which key of all those key (we have 10!) one should use.