Add markdown lint

[ajpl]

I noticed that there is no linter enabled for the Markdown files. Thus, if you forget to add a proper heading at the top of a md file you'd be "ruining" the README generation process.

I added a workflow that triggers on PRs with very relaxed rules:

• Enforce the first line to be a heading (more info)
• Restrict the line length to 120 chars (more info)

For a complete list of rules you can take a look over here.

Rules can be easily modified by changing the .markdownlint file.

Led me know if you find this useful or if you think the rules should be different.

Thanks!