[fractal] readme: Mention legacy branch

[Julian Sparber <jsparber src gnome org>]

commit c6cd1d7916494d3be55c4e7dc2581226d3baa6b1
Author: Kévin Commaille <zecakeh tedomum fr>
Date:   Thu Mar 31 12:06:55 2022 +0200

    readme: Mention legacy branch

 README.md | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)
---
diff --git a/README.md b/README.md
index fa8528775..c03a7cf0b 100644
--- a/README.md
+++ b/README.md
@@ -136,7 +136,9 @@ guidelines and paradigms. This will be revisited if or when GNOME gets a "Do Not
 Fractal-next is a complete rewrite of Fractal built on top of the
 [matrix-rust-sdk](https://github.com/matrix-org/matrix-rust-sdk) using [GTK4](https://gtk.org/).
 
-The previous version of Fractal was using GTK3 and its own backend to talk to a matrix homeserver.
+The previous version of Fractal was using GTK3 and its own backend to talk to a matrix homeserver,
+the code can be found in the [legacy](https://gitlab.gnome.org/GNOME/fractal/-/tree/legacy) branch.
+
 Initial versions were based on Fest <https://github.com/fest-im/fest>, formerly called ruma-gtk.
 In the origins of the project it was called guillotine, based on French revolution, in relation with
 the Riot client name, but it's a negative name so we decide to change for a math one.