tmux-cssh: add README

plugins/tmux-cssh/README.md

@@ -0,0 +1,10 @@
+# tmux-cssh plugin
+
+This plugin adds autocompletion for [`tmux-cssh`](https://github.com/zinic/tmux-cssh/).
+
+To use it, add `tmux-cssh` to the plugins array in your zshrc file:
+```zsh
+plugins=(... tmux-cssh)
+```
+
+First upstream repo, now disappeared: https://github.com/dennishafemann/tmux-cssh.