Expand on "features" section of readme

The "no JS" and "no cookies" portions of the readme warranted further
explanation. Since Whoogle uses JS and server-side cookies, it might be
confusing to a passerby what is actually meant by this. 

Note that both JS and cookies can be blocked and Whoogle will still be
able to perform searches perfectly well. 

Also updated the "theme" feature description

README.md

@@ -39,9 +39,9 @@ Contents
 
 ## Features
 - No ads or sponsored content
-- No JavaScript
-- No cookies
-- No tracking/linking of your personal IP address\*
+- No JavaScript\*
+- No cookies\*\*
+- No tracking/linking of your personal IP address\*\*\*
 - No AMP links
 - No URL tracking tags (i.e. utm=%s)
 - No referrer header
@@ -49,14 +49,18 @@ Contents
 - Autocomplete/search suggestions
 - POST request search and suggestion queries (when possible)
 - View images at full res without site redirect (currently mobile only)
-- Dark mode
+- Light/Dark/System theme modes (with support for [custom CSS theming](https://github.com/benbusby/whoogle-search/wiki/User-Contributed-CSS-Themes))
 - Randomly generated User Agent
 - Easy to install/deploy
 - DDG-style bang (i.e. `!<tag> <query>`) searches
 - Optional location-based searching (i.e. results near \<city\>)
-- Optional NoJS mode to disable all Javascript in results
+- Optional NoJS mode to view search results in a separate window with JavaScript blocked
 
-<sup>*If deployed to a remote server, or configured to send requests through a VPN, Tor, proxy, etc.</sup>
+<sup>*No third party JavaScript. Whoogle can be used with JavaScript disabled, but if enabled, uses JavaScript for things like presenting search suggestions.</sup>
+
+<sup>**No third party cookies. Whoogle uses server side cookies (sessions) to store non-sensitive configuration settings such as theme, language, etc. Just like with JavaScript, cookies can be disabled and not affect Whoogle's search functionality.</sup>
+
+<sup>***If deployed to a remote server, or configured to send requests through a VPN, Tor, proxy, etc.</sup>
 
 ## Dependencies
 If using Heroku Quick Deploy, **you can skip this section**.