knife_ssh: add README

plugins/knife_ssh/README.md

+# knife_ssh plugin
+
+This plugin adds a `knife_ssh` function as well as completion for it, to allow
+connecting via ssh to servers managed with [Chef](https://www.chef.io/).
+
+To use it, add `knife_ssh` to the plugins array in your zshrc file:
+```zsh
+plugins=(... knife_ssh)
+```
+
+The plugin creates a cache of the Chef node list via `knife`, and stores it
+in `$HOME/.knife_comp~`, when first triggering knife_ssh completion.
+
+**Requirements:** `knife` has to be installed.