[docs] Polymer features that aren't in LitElement

[rictic]

It looks like observers and computed functions are not supported (AIU #30 (comment)), and instead you just do some logic in either didRender or _render, which is reasonable enough, but it'd be nice for it to be documented.

I started with the decorators linked here: #7 (comment) and their typings mention observer and computed.

[PaulHMason]

Observers and computed functions aren't a feature of lit-element because they aren't required, so it doesn't really make sense to document that fact. The confusion stems from people comparing lit-element to a Polymer element, but they really aren't related and drawing comparisons will probably make things more confusing (especially to people who aren't familiar with Polymer).

[rictic]

LitElement uses a properties block in a very similar way to Polymer, and the README mentions that it leverages Polymer's properties mixins. A sentence or two clarifying what features of the properties block are supported / not supported would prevent confusion for users familiar with Polymer.

[akc42]

Yes, I just wasted a day chasing down the fact that camelcase propertied don't map to properties in the template with a "-" in them. Event though this was a lit element sitting inside a polymer 3 template

#128

[ernsheong]

query and queryAll decorators are also undocumented

[ghost]

Duplicate of https://github.com/Polymer/lit-element/issues/63