adds usage of env variables

Author: bradmartin

webpack.md

@@ -45,6 +45,40 @@ More env flags that are usually passed by the CLI automatically:
 - `--env.platform=<platform>` - for specifying the platform to use. Can be `android` or `ios`, or a custom platform in the future.
 - `--env.hmr` - `true` if building with HMR enabled
 
+#### Using flags at runtime
+
+In your app logic you can access `env` variables like the following:
+
+```js
+if (process.env.production) {
+  // this is a build executed with the `--env.production` flag
+  // typically this would be your release build
+} else {
+  // this is a debug/dev build
+}
+```
+
+::: tip Note
+If you want to check if you are in Dev or Release mode you can also use:
+
+```js
+if (__DEV__) {
+  // this is dev builds
+}
+```
+
+:::
+
+#### Using a .env file
+
+You can add a `.env` file to your project root and add any values to it and access those at runtime.
+
+```js
+if (process.env.VAR_NAME) {
+  // do work here if the variable is true
+}
+```
+
 ### CLI flags
 
 When running a NativeScript app the following flags have an effect on the webpack config: