Q:

Register a new namespace for autodiscovery

I'm using a vendor library that isn't managed through composer. I want to register the namespace for the library so that it will be discovered through PSR-4. I'm not sure how to go about doing this in D8 though. Can anyone point me at something?

A:

The autoload is still done using Composer. To add an extra namespace you'll have to run a composer command to rebuild the autoload file.
See https://stackoverflow.com/a/31505011/113057 for how to add a namespace to composer.
The key is in composer dump-autoload

