Add documentation about GOOGLE_APPLICATON_CREDENTIALS

[byrnereese]

We should add more verbose documentation about what the GOOGLE_APPLICATON_CREDENTIALS config parameter is for (transcription)!

To help developers who have never used this service, we should provide a little more handholding about how to sign up, and how to add credentials to their AI Bot, and what value to put in the .env file.

[byrnereese]

My PR #76 should resolve this.

[zxdong262]

@byrnereese Nice work!