Add metadata for these pages

These pages should get credited to Source Code. (I should also
check about CODEOWNERS entries too.)

README.md

+---
+stage: Create
+group: Source Code
+info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#assignments
+---
+
 # GitLab Shell
 
 ## GitLab Shell handles git SSH sessions for GitLab

doc/features.md

+---
+stage: Create
+group: Source Code
+info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#assignments
+---
+
 # Feature list
 
 ## Discover