Add todo item for OpenSearch.

[ikiwiki] / doc / todo / OpenSearch.mdwn

[[plugins/search]] could provide [OpenSearch](http://www.opensearch.org/)
metadata.  Various software supports OpenSearch (see the Wikipedia article on
[[wikipedia OpenSearch]]); in particular, browsers like Firefox and Iceweasel
will automatically discover an OpenSearch search and offer it in the search
box. --[[JoshTriplett]]