OpenSearch endpoint for Elgg

OVERVIEW
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OpenSearch is a standard for exposing the data in your website for search
to browsers and other web sites. For more information on this technology, visit
the Wikipedia page on OpenSearch at http://en.wikipedia.org/wiki/OpenSearch

INSTALL
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 * Rename config.ini.sample to config.ini
 * Edit config.ini to reflect your site
 * Activate the plugin

USAGE
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Adds an OpenSearch description definition to the html head. Browsers
will pick this up and allow you to add site specific search to your 
search bar.

The plugin also provides an RSS 2.0 feed that conforms to the OpenSearch
standard.