Merge pull request kubernetes#3443 from soltysh/fix_file_extension

Fix file extension

Author: k8s-ci-robot

keps/sig-cli/2229-kubectl-xdg-base-dir/README renamed to keps/sig-cli/2229-kubectl-xdg-base-dir/README.md

@@ -9,7 +9,14 @@
 - [Proposal](#proposal)
   - [Risks and Mitigations](#risks-and-mitigations)
 - [Design Details](#design-details)
+  - [Option 1: Replace .kubeconfig in loader.go](#option-1-replace-kubeconfig-in-loadergo)
+  - [Option 2: Set new RecommendedHomeFile to use XDG Base Directory Specification](#option-2-set-new-recommendedhomefile-to-use-xdg-base-directory-specification)
+  - [Option 3: Recommend users to use KUBECONFIG](#option-3-recommend-users-to-use-kubeconfig)
+  - [Additional info](#additional-info)
   - [Test Plan](#test-plan)
+    - [Alpha milestones](#alpha-milestones)
+    - [Beta milestones](#beta-milestones)
+    - [GA milestones](#ga-milestones)
   - [Graduation Criteria](#graduation-criteria)
     - [Alpha -> Beta Graduation](#alpha---beta-graduation)
     - [Beta -> GA Graduation](#beta---ga-graduation)