docs: add and enforce readme files for each manager

lib/manager/ansible-galaxy/readme.md

lib/manager/ansible/readme.md

lib/manager/buildkite/readme.md

lib/manager/bundler/readme.md

lib/manager/cargo/readme.md

lib/manager/cdnurl/readme.md

lib/manager/circleci/readme.md

lib/manager/composer/readme.md

lib/manager/deps-edn/readme.md

lib/manager/docker-compose/readme.md

lib/manager/dockerfile/readme.md

lib/manager/droneci/readme.md

lib/manager/git-submodules/readme.md

lib/manager/gitlabci-include/readme.md

lib/manager/gitlabci/readme.md

lib/manager/gradle/readme.md

lib/manager/helm-requirements/readme.md

lib/manager/helmfile/readme.md

lib/manager/homebrew/readme.md

lib/manager/html/readme.md