Added information about Ibexa Headless (#102)

Co-authored-by: Magdalena Dziadosz <35031941+MagdalenaZuba@users.noreply.github.com>

Author: webhdx

README.md

@@ -1,5 +1,7 @@
 # Ibexa DXP Content Symfony Flex metapackage
 
+⚠️ As of Ibexa DXP v4.6.0, Ibexa Content has been replaced by Ibexa Headless. Proceed to [ibexa/headless](https://github.com/ibexa/headless) to check out the latest version.
+
 This is a metapackage for Symfony Flex-based Ibexa DXP Content installation. 
 It requires a subscription.