[gnome-builder] update CONTRIBUTING.md

[Christian Hergert <chergert src gnome org>]

commit f59ec6b4e00165caef86d18dd9b44b535cdb6426
Author: Christian Hergert <chergert redhat com>
Date:   Mon Jun 19 15:29:24 2017 -0700

    update CONTRIBUTING.md

 CONTRIBUTING.md |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
---
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 986ebbc..2a7198d 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -101,6 +101,7 @@ please add a comment denoting such.
  - Update doc/conf.py to reflect the updated version number
  - Update any necessary tags in org.gnome.Builder.json for Flatpak
  - Make sure documentation builds, tests pass
+ - Make sure flatpak bundle builds
  - Commit release changes, add a signed tag (git tag -s -u $keyid)
  - Configure meson as normal, ensure docs are built
  - From the build directory, run `ninja dist` to generate the tarball