Mention that ready() and property observers won't fire until the element is added to the DOM

[justincy]

See Polymer/polymer#4526. That behavior is unexpected. The docs give no indication that it will behave that way.

The docs for ready() say

Called after property values are set and local DOM is initialized.

The docs on property observers say

The first call to an observer is deferred until the following criteria are met:

• The element is fully configured (default values have been assigned and data bindings propagated).
• At least one of the dependencies is defined (that is, they don't have the value undefined).

It should be mentioned somewhere that a v2 element isn't fully initialized until it's attached.