doc: describe skipping triggering a pipeline in Gitlab

'git push -o ci.skip' can be used to push to Gitlab without triggering a
pipeline.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/develop/ci_testing.rst b/doc/develop/ci_testing.rst
index b9a9a51..ffaaced 100644
--- a/doc/develop/ci_testing.rst
+++ b/doc/develop/ci_testing.rst
@@ -50,6 +50,12 @@
 pipeline on the free public instances provided at https://gitlab.com/ a problem
 with our squashfs tests currently prevents this.
 
+To push to Gitlab without triggering a pipeline use:
+
+.. code-block:: bash
+
+    git push -o ci.skip
+
 Docker container
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