Add docs file for environment variables

docs/docs/environment-variables.md

@@ -0,0 +1,29 @@
+---
+title: "Environment Variables"
+---
+
+You can easily provide environment variables to your site. Just add a `.env.development` and/or `.env.production` file in your root folder for development or production builds respectively. The environment variables are embedded during build time using Webpack's [DefinePlugin](https://webpack.js.org/plugins/define-plugin/). Because these variables are provided at build time, you will need restart your dev server or rebuild your site after changing them.
+
+## Example
+
+```
+# Example .env.development file
+
+API_URL=https://dev.example.com/api
+```
+
+```
+# Example .env.production file
+
+API_URL=https://example.com/api
+
+```
+
+These variables will be available to your site as `process.env.API_URL`.
+
+> You can not override certain environment variables as some are used internally for optimizations during build
+
+Reserved environment variables:
+
+- `NODE_ENV`
+- `PUBLIC_DIR`