[doc] Add reference to STSAFE-A_echo example in ReadMe.md

Author: Grom-

README.md

@@ -47,8 +47,12 @@ Reference implementation for most common STSE applicative use case can be found
 
 Following applicative project can be used as reference for STSELib integration and usage.
 
-- [STSAFE-A_device_authentication](https://github.com/STMicroelectronics/STSAFE-A_device_authentication)
-- [STSAFE-A_secure_data_storage](https://github.com/STMicroelectronics/STSAFE-A_secure_data_storage)
-- [STSAFE-A_asymmetric_cryptography](https://github.com/STMicroelectronics/STSAFE-A_asymmetric_cryptography)
-- [STSAFE-L_device_authentication](https://github.com/STMicroelectronics/STSAFE-L_device_authentication)
-- [STSAFE-L_secure_data_storage](https://github.com/STMicroelectronics/STSAFE-L_secure_data_storage)
+- STSAFE-A
+    - [STSAFE-A_echo](https://github.com/STMicroelectronics/STSAFE-A_echo) 
+    - [STSAFE-A_device_authentication](https://github.com/STMicroelectronics/STSAFE-A_device_authentication)
+    - [STSAFE-A_secure_data_storage](https://github.com/STMicroelectronics/STSAFE-A_secure_data_storage)
+    - [STSAFE-A_asymmetric_cryptography](https://github.com/STMicroelectronics/STSAFE-A_asymmetric_cryptography)
+  
+- STSAFE-L
+    - [STSAFE-L_device_authentication](https://github.com/STMicroelectronics/STSAFE-L_device_authentication)
+    - [STSAFE-L_secure_data_storage](https://github.com/STMicroelectronics/STSAFE-L_secure_data_storage)