[clutter] Update README.md

[Emmanuele Bassi <ebassi src gnome org>]

commit 25be50900b3c9745c4da45db347bd985b9f96c6f
Author: Emmanuele Bassi <ebassi gmail com>
Date:   Mon Oct 7 14:16:04 2019 +0000

    Update README.md
    
    Note that we're in deep maintenance mode, to avoid people submitting bugs or
    merge requests with new features.

 README.md | 12 ++++++++++++
 1 file changed, 12 insertions(+)
---
diff --git a/README.md b/README.md
index 0b30768a4..67159b65a 100644
--- a/README.md
+++ b/README.md
@@ -1,6 +1,18 @@
 Clutter
 =======
 
+Notice
+------
+
+Clutter is in deep maintenance mode; only micro releases addressing bug fixes
+are planned from now on. Additionally, the API and features are frozen.
+
+The planned replacement for Clutter is GTK 4.0.
+
+If you are fixing a bug in [Mutter](https://gitlab.gnome.org/GNOME/mutter) then
+please open a merge request against the internal copy of Clutter inside that
+project.
+
 What is Clutter?
 ----------------