Update README.md

Author: app-generator

README.md

@@ -166,10 +166,15 @@ Now, the API will start on port `5000`.
 **API Server URL** - `src/config/constant.js` 
 
 ```javascript
-const config = {
- ...
  API_SERVER: 'http://localhost:5001/api/' // <-- The magic line
-};
+```
+
+The value can be updated during the build using the environment
+
+```bash
+$ export REACT_APP_BACKEND_SERVER='http://localhost:5001/api/' # Unix/MacOS
+$ set REACT_APP_BACKEND_SERVER='http://localhost:5001/api/' # Windows CMD
+$ $env:REACT_APP_BACKEND_SERVER='http://localhost:5001/api/' # Windows PowerShell
 ```
 
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