indicate use of baseurl configuration for Github Pages deployment

[markwkidd]

I have been learning Jekyll and Just the Docs recently, thank you for this very helpful template. Based on my experience customizing Just the Docs, my intention with this PR is to bring the Just the Docs README and default configuration closer to the approach recommended by Jekyll.

If I understand Jekyll correctly, when a site is going to be served from a subfolder, the baseurl parameter should be set in _config.yml. Jekyll resources often specifically name a Github Pages deployment as an example of a situation when baseurl should be used.

So if I have a github repo named cushaw-handbook that I am deploying via Github Pages, I believe that the correct Jekyll configuration would include:

url: https://markwkidd.github.io
baseurl: cushaw-handbook

In my situation this is relevant to image processing and styling. With baseurl set, I can now use functionality Jekyll's relative_url function in my content:

![Drying seeds removed from fruit]( {{ '/assets/images/recipes/cut-fruit-with-seeds.jpg' | relative_url }} )

Thank you for considering this patch.