Unable to set source to use docs/

On _ <master> _ branch I already have a _ <docs/> _ folder, but the dropdown is not showing: 

Hi, @torrao!

There are two primary kinds of GitHub Pages sites: User Pages, and Project Pages. In most ways the two are identical, but there are subtle differences in how and where they are published.

Every GitHub account is entitled to a single User Pages site, located in a repository called username.github.io and accessible at https://username.github.io (unless a custom domain is configured). Sites in these repositories are automatically published from the master branch, and cannot be published from any other source.

Project Pages, on the other hand, can be enabled on any repository, and are accessible at https://username.github.io/repository-name (again, unless a custom domain is configured). These sites can be published from either the master branch, gh-pages branch, or the /docs folder on the master branch, but must be explicitly enabled by following the instructions in this help article:

https://help.github.com/articles/configuring-a-publishing-source-for-github-pages

In your case, it looks like your repository is a User Pages site. This is why there’s no option to publish it from the /docs folder on the master branch.

I hope that helps to explain!

Friday