please mdlinter in README.md

[bastelfreak]

this fixes: MD001 Header levels should only increment by one level at a time

Pull Request (PR) description

This Pull Request (PR) fixes the following issues

[bastelfreak]

it passes :thinking:

[baurmatt]

Should we modulesync a matching Travis job for this? We use the following internally in our Gitlab-CI:

markdown-lint:
  stage: test
  image: node:alpine
  before_script:
    - npm install -g markdownlint markdownlint-cli
  script: find . -type f -name "*\.md" -print0 | xargs -0 markdownlint

[bastelfreak]

in general I would like to have this. I'm using https://rubygems.org/gems/mdl at the moment. The issue is that none of our README.md passes, so enforcing this in a CI pipeline is tricky :(

[baurmatt]

@bastelfreak Can we just trigger a warning instead of a error? This might gradually fix it.