[gtk] README: Document nightly docs location

[Carlos Soriano <csoriano src gnome org>]

commit 580d96620e2238c77ef335dc2110efc3d55a8427
Author: Carlos Soriano <csoriano1618 gmail com>
Date:   Sun Aug 5 20:41:29 2018 +0000

    README: Document nightly docs location
    
    Useful for exposing the documentation early.

 README.md | 5 +++++
 1 file changed, 5 insertions(+)
---
diff --git a/README.md b/README.md
index 79c8183c99..aa13e92f9c 100644
--- a/README.md
+++ b/README.md
@@ -31,6 +31,11 @@ Information about mailing lists can be found at
 
   - http://www.gtk.org/mailing-lists.php
 
+Nightly documentation can be found at
+  - Gtk: https://gnome.pages.gitlab.gnome.org/gtk/gtk/
+  - Gdk: https://gnome.pages.gitlab.gnome.org/gtk/gtk/
+  - Gsk: https://gnome.pages.gitlab.gnome.org/gtk/gsk/
+
 Building and installing
 -----------------------