[gimp-macos-build] Add information about the mirror

[Alex Samorukov <asamorukov src gnome org>]

commit 170924b1944e464ff7c427a175dd0136ebe19262
Author: Oleksii Samorukov <oleksii_samorukov mckinsey com>
Date:   Fri Aug 30 18:21:36 2019 +0200

    Add information about the mirror

 README.md | 2 ++
 1 file changed, 2 insertions(+)
---
diff --git a/README.md b/README.md
index 7f8cc85..5b5c576 100644
--- a/README.md
+++ b/README.md
@@ -8,6 +8,8 @@ To build GIMP/MacOS we are using [fork](https://gitlab.gnome.org/samm-git/gtk-os
 of the [gtk-osx](https://gitlab.gnome.org/GNOME/gtk-osx) project. Fork adds modules related to GIMP
 and some gimp-specific patches to GTK. Currently build is done using CircleCI.
 
+Because CircleCI is not supporting gitlab [yet] there is a [GitHub 
mirror](https://github.com/GNOME/gimp-macos-build) of this repository.
+
 ### Steps in the [CircleCI 
config.yml](https://github.com/GNOME/gimp-macos-build/blob/master/.circleci/config.yml) are:
 
 - Install gfortran and rust as they are required for the GIMP dependencies.