Add example demonstrating setting DOTENV_KEY

Author: motdotla

README.md

@@ -243,6 +243,7 @@ See [examples](https://github.com/dotenv-org/examples) of using dotenv with vari
 * [nodejs (debug on)](https://github.com/dotenv-org/examples/tree/master/dotenv-nodejs-debug)
 * [nodejs (override on)](https://github.com/dotenv-org/examples/tree/master/dotenv-nodejs-override)
 * [nodejs (processEnv override)](https://github.com/dotenv-org/examples/tree/master/dotenv-custom-target)
+* [nodejs (DOTENV_KEY override)](https://github.com/dotenv-org/examples/tree/master/dotenv-vault-custom-target)
 * [esm](https://github.com/dotenv-org/examples/tree/master/dotenv-esm)
 * [esm (preload)](https://github.com/dotenv-org/examples/tree/master/dotenv-esm-preload)
 * [typescript](https://github.com/dotenv-org/examples/tree/master/dotenv-typescript)