Document required credentials

[robmarkcole]

Running python examples/summary_stack.py I get

AttributeError: You need to hav env : EDS_AUTH_URL, EDS_SECRET and EDS_CLIENT_ID

I recommend using dotenv for managing these, allows providing a template that can be edited and gitignored

With dotenv installed, user creates a .env file with contents:

EDS_AUTH_URL="https://.."
EDS_CLIENT_ID="123"
EDS_SECRET="123"

Then in the script we add:

from dotenv import load_dotenv

load_dotenv()  # take environment variables from .env.

Validated

[robmarkcole]

The changes from https://github.com/GEOSYS/earthdaily-python-client/pull/8 also snuck in on my branch