This lightweight module creates an opensearch autodiscovery link on every page of your site, as described here.  It can serve the description document from a static file, but if no such file exists, it will create a dynamic one with some potentially reasonable defaults.

By default, the description document is located at /opensearch.xml, and is generated dynamically. To use a static file, copy or rename the opensearch_link/example.xml file to opensearch_link/opensearch.xml and edit it to suit your site.  The module looks for that file and uses it automatically if it exists.