[gjs: 1/2] Update CONTRIBUTING.md about the runner system failure

[Philip Chimento <pchimento src gnome org>]

commit d966cf8abf5e2ef8e755785896613f2c2a5ec39f
Author: Nasah-Kuma <nasahnash19 gmail com>
Date:   Mon Oct 26 19:29:46 2020 +0000

    Update CONTRIBUTING.md about the runner system failure

 CONTRIBUTING.md | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)
---
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index f24c66a2..d34e3b8f 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -48,7 +48,8 @@ Your responsibilities as a contributor:
   an issue in the [bugtracker], discuss things transparently, and get
   community feedback.
 - Ensure all jobs are green on GitLab CI for your merge requests.
-- Your code must pass the tests.
+- Your code must pass the tests. Sometimes you can experience a runner
+  system failure which can be fixed by re-running the job.
 - Your code must pass the linters; code should not introduce any new
   linting errors.
 - Your code should not cause any compiler warnings.