[valadoc] README: Add notice about migration to Vala repo

[Rico Tzschichholz <ricotz src gnome org>]

commit 5e3f5269500e310d7bdc22a77121c941b23a2014
Author: Nahuel Gomez Castro <nahual_gomca outlook com ar>
Date:   Thu Jul 29 14:33:43 2021 -0300

    README: Add notice about migration to Vala repo

 README | 6 ++++++
 1 file changed, 6 insertions(+)
---
diff --git a/README b/README
index e69de29b..dd9b7065 100644
--- a/README
+++ b/README
@@ -0,0 +1,6 @@
+NOTICE: valadoc got merged into Vala itself since 0.38.
+
+This repo was only needed for Vala <= 0.36, which is currently EOL. Please send
+your contributions to https://gitlab.gnome.org/GNOME/vala instead.
+
+See https://bugzilla.gnome.org/show_bug.cgi?id=782782 for more details.