[doc] readme updates

README.rst

     -  opensearch/rss ``curl https://searx.0x2a.tk/?format=rss&q=[query]``
 -  Opensearch support (you can set as default search engine)
 -  Configurable search engines/categories
+-  Different search languages
 -  Duckduckgo like !bang functionality with engine shortcuts
 -  Parallel queries - relatively fast