To add libraries, navigate to Libraries Code, select New Library, then copy/paste or import code in a manner similar to that which you would do for app or driver code.
If you do not see Libraries Code in your sidebar, click or tap the small arrow next to Developer Tools to expand the list.
If you are a developer looking to write library code (shared pieces of code that may be used in any number of apps and/or drivers), consult the developer documentation.