Update README.md with a deprecation message

README.md

 # OTB provider for QGIS Processing
 
-**OTB provider will be available in processing core starting from qgis 3.8.**
+## This plugin is deprecated
 
+Since QGIS 3.8, this plugin is part of QGIS’s source code. It is not required anymore. 
+
+If you are using an older versions of QGIS, you still need this plugin. But those old versions of QGIS are 
+deprecated (QGIS 3.6 went out 2 years ago), and you should consider to upgrade to a more recent version.
 
 ## Version
 * QGIS : 3.2.0 or later