werc.cat-v.org: document duckduckgo app

sites/werc.cat-v.org/apps/duckduckgo/index.md

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+duckduckgo - Simple sitesearch using DuckDuckGo.com
+===================================
+
+duckduckgo is a very simple app that allows you to redirect a path 
+(by default /_search/) to a query to duckduckgo.com restricted to 
+your site.
+
+Currently, to change that path requires editing the app.  In the
+future, the path will be a configuration option.
+
+Configuration
+-------------
+
+First, create the search path and the _werc directory under it:
+
+; mkdir -p /www/werc/sites/MYSITE/_search/_werc/
+
+Then, enable the duckduckgo app in that directory:
+
+echo 'conf_enable_duckduckgo' > /www/werc/sites/MYSITE/_search/_werc/config
+
+Next, deploy the search form somewhere on your site.  This example puts
+it in your footer:
+
+mkdir -p /www/werc/sites/MYSITE/_werc/lib/
+cp /www/werc/apps/search/footer.inc.sample /www/werc/sites/MYSITE/_werc/lib/footer.inc
+
+Demo
+----
+
+For a demo of this app, see the footer of your favorite cat-v.org site.
+
+TODO
+----
+
+* Make the search path a configuration option
+* Provide a template for non-footer deployment
+* Enable the search path itself to serve a search form to GET requests
+