oSearch: Search via OpenSearch by seril🍋

1. Add OpenSearch Description Document (OSDD) for the site supported, so that the Firefox can recognize that this site exists search engine
2. After Firefox recognizes the search engine, click on the website icon with a plus sign in the search bar or address bar to add it into the search engine.
3. Done! You can simply search the content on the website by either setting a search shortcut or by entering the URL in the address bar and selecting search with it.