Add proxy example

Author: Laurynas Butkus

README.md

@@ -33,6 +33,16 @@ import convertapi
 convertapi.api_secret = 'your-api-secret'
 ```
 
+#### Proxy configuration
+
+If you need to use proxy, you specify it using `HTTPS_PROXY` environment variable when running your script.
+
+Example:
+
+```
+CONVERT_API_SECRET=secret HTTPS_PROXY=https://user:pass@127.0.0.1:9000/ python convert_word_to_pdf_and_png.py
+```
+
 ### File conversion
 
 Example to convert file to PDF. All supported formats and options can be found