Mention ladsweb config file in README

[dctanner]

Hey, incredible work!

It took me a while to work out where to save the ladsweb auth token. Maybe it's worth adding something like this to the readme (taken from https://pypi.org/project/geoget/):

# To create an account and an authentication token visit ladsweb website.
{
    "url"   : "https://ladsweb.modaps.eosdis.nasa.gov",
    "key"   : "",
    "email" : ""
}