[gtksourceview] README: link to documentation

[Christian Hergert <chergert src gnome org>]

commit fb185953568e9b78e2dd5efbc552aa7f97ab3e94
Author: Christian Hergert <chergert redhat com>
Date:   Wed Mar 10 14:28:04 2021 -0800

    README: link to documentation

 README.md | 5 +++++
 1 file changed, 5 insertions(+)
---
diff --git a/README.md b/README.md
index 2fcd2dcf..fa723ab5 100644
--- a/README.md
+++ b/README.md
@@ -11,6 +11,11 @@ The GtkSourceView library is free software and is released under the terms of
 the GNU Lesser General Public License, see the 'COPYING' file for more details.
 The official web site is https://wiki.gnome.org/Projects/GtkSourceView.
 
+Documentation
+-------------
+
+Nightly documentation can be [found 
here](https://gnome.pages.gitlab.gnome.org/gtksourceview/gtksourceview-5.0/).
+
 Dependencies
 ------------