bwana: add README (#7369)

plugins/bwana/README.md

+# Bwana
+
+This plugin provides a function to open `man` pages directly with [Bwana](https://www.bruji.com/bwana/).
+
+To use it add bwana to the plugins array in your zshrc file.
+
+```bash
+plugins=(... bwana)
+```