Updating index.md

Author: github-actions[bot]

index.md

@@ -1,6 +1,7 @@
 # Arduino core for the ESP32, ESP32-S2, ESP32-S3 and ESP32-C3
 
 ![Build Status](https://github.com/espressif/arduino-esp32/workflows/ESP32%20Arduino%20CI/badge.svg) [![Documentation Status](https://readthedocs.com/projects/espressif-arduino-esp32/badge/?version=latest)](https://docs.espressif.com/projects/arduino-esp32/en/latest/?badge=latest)
+[![External Libraries Test](https://github.com/espressif/arduino-esp32/actions/workflows/lib.yml/badge.svg?branch=master&event=schedule)](https://github.com/espressif/arduino-esp32/actions/workflows/lib.yml?link=http://https://github.com/espressif/arduino-esp32/blob/master/LIBRARIES_TEST.md)
 
 ### Need help or have a question? Join the chat at [Gitter](https://gitter.im/espressif/arduino-esp32) or [open a new Discussion](https://github.com/espressif/arduino-esp32/discussions)
 
@@ -22,7 +23,7 @@ Latest Development Release  [![Release Version](https://img.shields.io/github/re
 
 ### Development Planning
 
-Our Development is fully tracked on this public **[Roadmap 🎉](https://github.com/orgs/espressif/projects/3)** 
+Our Development is fully tracked on this public **[Roadmap 🎉](https://github.com/orgs/espressif/projects/3)**
 
 For even more information you can take a look at [Sprint Meeting notes](https://github.com/espressif/arduino-esp32/discussions/categories/sprints-meeting-notes) or join [Monthly Community Meetings 🔔](https://github.com/espressif/arduino-esp32/discussions/categories/monthly-community-meetings)
 
@@ -51,6 +52,11 @@ Before reporting an issue, make sure you've searched for similar one that was al
 
 Finally, if you are sure no one else had the issue, follow the **Issue template** or **Feature request template** while reporting any [new Issue](https://github.com/espressif/arduino-esp32/issues/new/choose).
 
+### External libraries compilation test
+
+We have set-up CI testing for external libraries for ESP32 Arduino core. You can check test results in the file [LIBRARIES_TEST](https://github.com/espressif/arduino-esp32/blob/gh-pages/LIBRARIES_TEST.md).
+For more information and how to add your library to the test see [external library testing](https://docs.espressif.com/projects/arduino-esp32/en/latest/external_libraries_test.html) in the documentation.
+
 ### Contributing
 
 We welcome contributions to the Arduino ESP32 project!