Update the Polymer/xxx 0.5 element's README

[notwaldorf]

They're all currently saying something along the lines of "This element is compatible with Polymer 0.5 and lower only, and will be deprecated. You can check out a similar 0.8-compatible version of this element " (see https://github.com/Polymer/paper-button), which is pretty lulz. 😂

/cc @tjsavage