Update README instructions for copying spreadsheet

[acharliekelly]

The instructions for creating your own spreadsheet don't work - copying the spreadsheet works, but since we started using the Directory, the API no longer takes the code for a single sheet. So, if you have to do database testing, I think you need write access to one of the test pages on the directory. This should be explained on the README.

[acharliekelly]

Oh, actually, never mind

[acharliekelly]

OK, now I'm not sure whether this is an issue or not.

Using the environment variable REACT_APP_GOOGLE_SHEETS_ID may or may not work - I can't get it to run locally, so I can't test it. But there is code in AppContainer.js that should use the sheet ID, if one is provided.
Does anyone know if this actually works?

[acharliekelly]

We don't need the environment variable anymore. Just put the new spreadsheet on the Directory, and then point your app to that resource's path. But it's possible someone might still want to use the env option.