How to create a Resource Root folder?

According to this, Resource Root folders are:

[...] are intended for resource files in your application (images, style sheets, and so on). By assigning a folder to this category, you tell PyCharm that files in it and in its subfolders can be referenced relative to this folder instead of specifying full paths to them.

My question is how to mark a folder as a Resource Root in PyCharm? When I right click the folder I want, and I go to Mark Directory as, I can see only "Sources Root" and "Excluded".