ikiwiki's signin page is too sparse for people who don't live in the Web 2.0.

We occasionally have GNU Hurd web pages contributors wonder what they have to
do on that page.  They don't know / the page doesn't explain what an *account
provider* is, and that cross-indentification (using an existing OpenID account)
is possible to begin with.  And, if they don't have such an OpenID account,
it's not easily understandable that the *other* option is for creating a local
site-only account (like in the old days).

--[[tschwinge]]

> I agree that this would be good. It could be done by leaving
> the compact widget at the top and adding some verbose explanations
> and/or further forms below. 
> 
> All it takes is editing `templates/openid-selector.tmpl`,
> so I welcome suggestions. --[[Joey]] 
