torrent: add README (#7873)

plugins/torrent/README.md

+# torrent
+
+This plugin creates a Torrent file based on a [MagnetURI](https://en.wikipedia.org/wiki/Magnet_URI_scheme).
+
+To use it, add `torrent` to the plugins array in your zshrc file.
+
+```zsh
+plugins=(... torrent)
+```
+
+## Plugin commands
+
+* `magnet_to_torrent <MagnetURI>`: creates Torrent file.