[gnome-shell-sass] docs: Clarify where style changes should be done

[Florian Müllner <fmuellner src gnome org>]

commit 6cd1fc7f4ab63b06d552de4936e2190205a91171
Author: Florian Müllner <fmuellner gnome org>
Date:   Wed May 23 18:30:20 2018 +0200

    docs: Clarify where style changes should be done
    
    After the move to Gitlab, the gnome-shell-sass repository is more
    exposed than it used to; clarify that it is not the place where
    style changes should happen, but rather the corresponding subtree
    in the gnome-shell repository.

 README.md |    6 ++++++
 1 files changed, 6 insertions(+), 0 deletions(-)
---
diff --git a/README.md b/README.md
index 7d6090f..a5f219a 100644
--- a/README.md
+++ b/README.md
@@ -3,8 +3,14 @@ GNOME Shell Sass is a project intended to allow the sharing of the
 theme sources in sass between gnome-shell and other projects like
 gnome-shell-extensions.
 
+Any changes should be done in the [GNOME Shell subtree][shell-subtree]
+and not the stand-alone [gnome-shell-sass repository][sass-repo]. They
+will then be synchronized periodically before releases.
+
 ## License
 GNOME Shell Sass is distributed under the terms of the GNU General Public
 License, version 2 or later. See the [COPYING][license] file for details.
 
+[shell-subtree]: https://gitlab.gnome.org/GNOME/gnome-shell/tree/master/data/theme/gnome-shell-sass
+[sass-repo]: https://gitlab.gnome.org/GNOME/gnome-shell-sass
 [license]: COPYING