Add a “contrib” directory to your projects

I think this only works with the caveat that the core code is so well-designed that the user is capable of mixing contrib APIs with core APIs without requiring action on part of core developers.

For instance, TensorFlow v1 had a contrib directory that encouraged a lot of internal contributions and features to be added. This was initially quite nice due to the "batteries included" argument. However, the success of TF quickly resulted in a lot of bloat being added. There were competing ways to do the same thing within the default TF installation, and the worst part was that users expected contrib features to be interoperable with all other contrib features and core TF.

For this to work, contrib must be possible out-of-build/tree, which is a good thing, because it means there can be other "contrib" repositories as well - not just the single special directory in the mainline repo.