Add docs when using in a Typescript project

[Clumsy-Coder]

Solves the issue of Typescript not finding the named environment variable import from the npm package.

Solution obtained from https://github.com/zetachang/react-native-dotenv/issues/76#issuecomment-585171009

[Clumsy-Coder]

Had to close and reopen pull request.

Has a weird issue with CircleCI.

Got error message:

No configuration was found in your project.

[goatandsheep]

There's a new repo and package babel-plugin-dotenv-import. Please reopen in the new repo