[LyX/master] Add note about deprecated layouts.

[Pavel Sanda]

commit 3e97b90b40fd71b4d3e4395c8efaa9853c3f6d0b
Author: Pavel Sanda <sanda at lyx.org>
Date:   Wed Jan 4 22:13:38 2023 +0100

    Add note about deprecated layouts.
---
 lib/layouts/TODO.txt |    9 +++++++++
 1 files changed, 9 insertions(+), 0 deletions(-)

diff --git a/lib/layouts/TODO.txt b/lib/layouts/TODO.txt
new file mode 100644
index 0000000..691698e
--- /dev/null
+++ b/lib/layouts/TODO.txt
@@ -0,0 +1,9 @@
+These layouts are ripe to be moved into some form of attic once we decide how
+to handle old documents using such layouts.
+
+The reason for deprecation is that the relevant cls files have not been
+available for several years (see kluwer related thread from 06 Dec 2022).
+
+iopart.layout
+kluwer.layout
+svjog.layout