Update README with merge information

Added important notice about repository merge.

Author: t3chguy

README.md

@@ -1,3 +1,6 @@
+> [!IMPORTANT]
+> This repository has been merged back into https://github.com/element-hq/element-web within `apps/desktop` subdirectory
+
 ![Build](https://github.com/vector-im/element-desktop/actions/workflows/build.yaml/badge.svg)
 ![Static Analysis](https://github.com/vector-im/element-desktop/actions/workflows/static_analysis.yaml/badge.svg)
 [![Localazy](https://img.shields.io/endpoint?url=https%3A%2F%2Fconnect.localazy.com%2Fstatus%2Felement-web%2Fdata%3Fcontent%3Dall%26title%3Dlocalazy%26logo%3Dtrue)](https://localazy.com/p/element-web)