Requirement to prefix env variables with "REACT_APP_" not documented

[randyxli]

It's unclear from the documentation that environment variables must be prefixed with REACT_APP_ for this package to work. I feel this behavior is not desirable in cases where an environment variable should configured on both the frontend and the backend, such as an OAuth client ID. One benefit of only setting variables with that prefix is it avoids the unintentional leaking environment variables, but I think that can be circumvented by defining variables to be distributed in env.js explicitly. Either way, the current prefix requirement should be documented. Happy to submit a PR once its clear that the repo is being maintained.

[sqxieshuai]

you saved my life!