Add PR template with doc checklist (#1470)

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,15 @@
+<!--  Thanks for sending a pull request! Here are some tips for you:
+1. If this is your first time, check our contributor guidelines: https://www.kubeflow.org/docs/about/contributing
+2. To know more about Training Operator, check the developer guide:
+    https://github.com/kubeflow/training-operator/blob/master/docs/development/developer_guide.md
+3. If you want *faster* PR reviews, check how: https://git.k8s.io/community/contributors/guide/pull-requests.md#best-practices-for-faster-reviews
+-->
+
+**What this PR does / why we need it**:
+
+**Which issue(s) this PR fixes** _(optional, in `Fixes #<issue number>, #<issue number>, ...` format, will close the issue(s) when PR gets merged)_:
+Fixes #
+
+**Checklist:**
+
+- [ ] [Docs](https://www.kubeflow.org/docs/components/training/) included if any changes are user facing