[doc] features update

README.md

 * Modular (see [examples](https://github.com/asciimoo/searx/blob/master/examples))
 * Parallel queries
 * Supports json output `curl -d 'format=json' -d 'q=searx' http://searx.0x2a.tk/`
+* Opensearch support (you can set as default search engine in your browser)
 
 ### Installation