Clarify relationship between polymer-elements and core-elements packages.

[GirlBossRush]

It's not clear what the difference, if any, there is between polymer-elements and core-elements. The documentation vaguely states that polymer-elements is experimental. I'd prefer if this concept was merged core-elements and experimental elements are not included in the bundle until they are ready for general use. This allows users to include core-untested-element into their project without affecting anyone else.

Thank you.

[ebidel]

@mattsmcnulty posted info on the mailing list:
https://groups.google.com/forum/?utm_medium=email&utm_source=footer#!msg/polymer-dev/HUqNk31y3EY/8AXDzQeBQ8sJ

To clarify about element "bundles"...we have shim repos that aggregate a list of elements:
https://github.com/polymer/core-elements
https://github.com/polymerlabs/polymer-elements
https://github.com/polymerlabs/polymer-ui-elements
https://github.com/PolymerLabs/more-elements

Those essentially make it easy to bower install a bunch of related elements all in one.

In general, you should use core-* whenever possible. These are the elements that we're productionizing, documenting, and support.

FWIW, we are missing documentation for some of the core-* elements and the corpus is continuing to grow. When it's more polished, we'll deemphasis the old stuff even more.