The addition of a /libraries folder (like /modules and /themes) for the purpose of contrib. and project specific custom modules to place shareable javascript or css libraries that should not be included in a module or theme repos due to licensing restrictions and policies of Drupal; is necessary.
In light of 3rd party libraries and content on Drupal.org module and theme developers need a location to instruct Site Builders to download and install said library to.
README should contain:
- why this is folder is here
- notes/links on use of *.libraries.yml file in module/theme to access said library.
- For modules: and installation process points with hook_install (*.install file) and use of hook_requirements($phase) to check the 'install' phase -- demo code
- extended use with Libraries API 3.x -- hook_library_info()