Add note about automatic pipelines when enabling Auto DevOps

doc/topics/autodevops/index.md

@@ -146,6 +146,10 @@ has a `.gitlab-ci.yml` or not:
   All you need to do is remove your existing `.gitlab-ci.yml`, and you can even
   do that in a branch to test Auto DevOps before committing to `master`.
+NOTE: **Note:**
+Starting with GitLab 10.3, when enabling Auto DevOps, a pipeline is
+automatically run on the default branch.
+
 NOTE: **Note:**
 If you are a GitLab Administrator, you can enable Auto DevOps instance wide
 in **Admin Area > Settings > Continuous Integration and Deployment**. Doing that,