clarifying the use of custom environment variables (facebook#1513)

* clarifying the use of custom environment variables

* Tweak

packages/react-scripts/template/README.md

@@ -495,7 +495,7 @@ Now you are ready to use the imported React Bootstrap components within your com
 
 Flow is a static type checker that helps you write code with fewer bugs. Check out this [introduction to using static types in JavaScript](https://medium.com/@preethikasireddy/why-use-static-types-in-javascript-part-1-8382da1e0adb) if you are new to this concept.
 
-Recent versions of [Flow](http://flowtype.org/) work with Create React App projects out of the box. 
+Recent versions of [Flow](http://flowtype.org/) work with Create React App projects out of the box.
 
 To add Flow to a Create React App project, follow these steps:
 
@@ -515,7 +515,11 @@ To learn more about Flow, check out [its documentation](https://flowtype.org/).
 
 Your project can consume variables declared in your environment as if they were declared locally in your JS files. By
 default you will have `NODE_ENV` defined for you, and any other environment variables starting with
-`REACT_APP_`. These environment variables will be defined for you on `process.env`. For example, having an environment
+`REACT_APP_`.
+
+>Note: You must create custom environment variables beginning with `REACT_APP_`. Any other variables except `NODE_ENV` will be ignored to avoid accidentally [exposing a private key on the machine that could have the same name](https://github.com/facebookincubator/create-react-app/issues/865#issuecomment-252199527).
+
+These environment variables will be defined for you on `process.env`. For example, having an environment
 variable named `REACT_APP_SECRET_CODE` will be exposed in your JS as `process.env.REACT_APP_SECRET_CODE`, in addition
 to `process.env.NODE_ENV`.
 
@@ -623,12 +627,12 @@ These tutorials will help you to integrate your app with an API backend running
 using `fetch()` to access it.
 
 ### Node
-Check out [this tutorial](https://www.fullstackreact.com/articles/using-create-react-app-with-a-server/). 
+Check out [this tutorial](https://www.fullstackreact.com/articles/using-create-react-app-with-a-server/).
 You can find the companion GitHub repository [here](https://github.com/fullstackreact/food-lookup-demo).
 
 ### Ruby on Rails
 
-Check out [this tutorial](https://www.fullstackreact.com/articles/how-to-get-create-react-app-to-work-with-your-rails-api/). 
+Check out [this tutorial](https://www.fullstackreact.com/articles/how-to-get-create-react-app-to-work-with-your-rails-api/).
 You can find the companion GitHub repository [here](https://github.com/fullstackreact/food-lookup-demo-rails).
 
 ## Proxying API Requests in Development
@@ -961,7 +965,7 @@ Snapshot testing is a feature of Jest that automatically generates text snapshot
 
 ### Editor Integration
 
-If you use [Visual Studio Code](https://code.visualstudio.com), there is a [Jest extension](https://github.com/orta/vscode-jest) which works with Create React App out of the box. This provides a lot of IDE-like features while using a text editor: showing the status of a test run with potential fail messages inline, starting and stopping the watcher automatically, and offering one-click snapshot updates. 
+If you use [Visual Studio Code](https://code.visualstudio.com), there is a [Jest extension](https://github.com/orta/vscode-jest) which works with Create React App out of the box. This provides a lot of IDE-like features while using a text editor: showing the status of a test run with potential fail messages inline, starting and stopping the watcher automatically, and offering one-click snapshot updates.
 
 ![VS Code Jest Preview](https://cloud.githubusercontent.com/assets/49038/20795349/a032308a-b7c8-11e6-9b34-7eeac781003f.png)
 
@@ -1196,15 +1200,15 @@ GitHub Pages doesn't support routers that use the HTML5 `pushState` history API
 ### Heroku
 
 Use the [Heroku Buildpack for Create React App](https://github.com/mars/create-react-app-buildpack).<br>
-You can find instructions in [Deploying React with Zero Configuration](https://blog.heroku.com/deploying-react-with-zero-configuration). 
+You can find instructions in [Deploying React with Zero Configuration](https://blog.heroku.com/deploying-react-with-zero-configuration).
 
 #### Resolving "Module not found: Error: Cannot resolve 'file' or 'directory'"
 
 Sometimes `npm run build` works locally but fails during deploy via Heroku with an error like this:
 
 ```  
 remote: Failed to create a production build. Reason:
-remote: Module not found: Error: Cannot resolve 'file' or 'directory' 
+remote: Module not found: Error: Cannot resolve 'file' or 'directory'
 MyDirectory in /tmp/build_1234/src  
 ```