How to place a library in DRUPAL_ROOT/libraries?
In drupal 7 the shared libraries were provided by a contrib module. Because a lot of modules had this as a dependency, it was very common:

Libraries API

I don't know, if this will be used this often in drupal 8, you can follow the development here:

https://www.drupal.org/node/1704734

So at the moment you are stuck with the libraries in *.libraries.yml, when you don't want to use the dev version of the Libraries API.

The libraries in *.libraries.yml were not designed for libraries in a shared directory. But they have one very useful feature, you can borrow a library from another module. There are a lot of useful packages already in core modules, like jquery ui elements. If you define a library in one of your modules, they also become available for all other modules. But the files will remain local to that module, which defines the library.