Document current WebUI browser compatibility

src/webui/www/README.md

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+qBittorrent Web UI
+---
+
+### Browser compatibility
+
+| Browser           | Lower bound                                        |
+| ----------------- | -------------------------------------------------- |
+| Chrome            | [The release from 1 year ago][Chrome-history-link] |
+| Firefox           | [Oldest active ESR release][Firefox-ESR-link]      |
+| Internet Explorer | 11                                                 |
+| Microsoft Edge    | [The release from 1 year ago][MSEdge-history-link] |
+| Safari            | [The release from 1 year ago][Safari-history-link] |
+
+The upper bound will always be the latest stable release.
+
+[Chrome-history-link]: https://en.wikipedia.org/wiki/Google_Chrome_version_history
+[Firefox-ESR-link]: https://en.wikipedia.org/wiki/Firefox_version_history#Current_and_future_releases
+[MSEdge-history-link]: https://en.wikipedia.org/wiki/Microsoft_Edge#Release_history
+[Safari-history-link]: https://en.wikipedia.org/wiki/Safari_version_history