[doc] output formats

README.md

 * Tracking free
 * Modular (see [examples](https://github.com/asciimoo/searx/blob/master/examples))
 * Parallel queries
-* Supports json output `curl https://searx.0x2a.tk/?format=json&q=[query]`
-* Supports csv output `curl https://searx.0x2a.tk/?format=csv&q=[query]`
+* Supports multiple output formats
+ * json `curl https://searx.0x2a.tk/?format=json&q=[query]`
+ * csv `curl https://searx.0x2a.tk/?format=csv&q=[query]`
+ * opensearch/rss `curl https://searx.0x2a.tk/?format=rss&q=[query]`
 * Opensearch support (you can set as default search engine)
 * Configurable search engines/categories