Update README.md (#259)

* Update README.md

I got some errors about failed API calls and guessed this might work. It seems to work. There may be a better way to document it, but something's better than nothing, hopefully!

* Update README.md

Linked to docs on getting an API key

* Update README.md

DC_API_KEY is now `DC_API_KEY` (backticks for code)

* Update README.md

referencing v2 API docs for auth now, not v1

simple/README.md

@@ -13,6 +13,8 @@ python3 -m stats.main
 By default it reads inputs CSVs from the `.data/input` folder and outputs
 a sqlite `datacommons.db` file in the `.data/output` folder.
 
+To enable Data Commons API lookups, set a `DC_API_KEY` environment variable. See [API documentation](https://docs.datacommons.org/api/rest/v2/getting_started#authentication) to learn more about getting and using API keys.
+
 ## Other options
 
 To see all parameters and overrides supported by the script: