Add warning for the new SDK system

Author: Mickhopes

README.md

@@ -1,3 +1,7 @@
+**CAUTION: this git repository is no longer updated. The project has been merged into the linphone-sdk ([Gitlab](https://gitlab.linphone.org/BC/public/linphone-sdk), [Github](https://github.com/BelledonneCommunications/linphone-sdk)) git repository and will continue his life there.**
+
+**Versions up to 5.4 (included) are still kept into this repository, that will remain active on release/5.4 branch until the end of life of release 5.4.**
+
 [![pipeline status](https://gitlab.linphone.org/BC/public/mediastreamer2/badges/master/pipeline.svg)](https://gitlab.linphone.org/BC/public/mediastreamer2/commits/master)
 
 Mediastreamer2