chore: Add deprecation notice to README.md

We plan to decommission this repository, let's add a note at the top of
the README.md to indicate why.

Signed-off-by: Quentin Monnet <qmo@qmon.net>

Author: qmonnet

README.md

@@ -1,3 +1,8 @@
+> [!WARNING]
+> ### DPDK-SYS DECOMMISSION
+>
+> This repository is deprecated now that the dataplane repository has switched to its new build framework (https://github.com/githedgehog/dataplane/pull/1304).
+
 # dpdk-sys
 
 [Nix][`nix`] build suite for a rust + dpdk toolchain.