README: Mark 1.10.x as an end-of-life branch

Signed-off-by: Simon McVittie <smcv@collabora.com>

README.md

@@ -1,3 +1,17 @@
+# Flatpak 1.10.x: end-of-life branch
+
+The Flatpak 1.10.x branch is no longer supported and does not receive
+security fixes.
+Please upgrade to a supported branch as documented in
+[SECURITY.md](https://github.com/flatpak/flatpak/blob/main/SECURITY.md).
+
+If maintainers of long-term-supported OS distributions are still
+distributing Flatpak 1.10.x, those vendors will need to either take
+responsibility for backporting security fixes, or upgrade their Flatpak
+packages.
+
+---
+
 <p align="center">
   <img src="https://github.com/flatpak/flatpak/blob/master/flatpak.png?raw=true" alt="Flatpak icon"/>
 </p>