Add README for updating .gitignore templates

vendor/gitignore/README.md

+# .gitignore templates
+
+This directory contains language-specific .gitignore templates that are used by GitLab.
+
+These files were automatically pulled from [this repository](https://github.com/github/gitignore).
+Please submit pull requests to that repository. There is no need to edit the files in this directory.
+
+## Bulk Update
+
+To update this directory with the latest changes in the repository, run:
+
+```sh
+bundle exec rake gitlab:update_gitignore
+```