knife: add README and reformat completion file

plugins/knife/README.md

+# knife plugin
+
+This plugin adds completion for [knife](https://docs.chef.io/knife.html), a command-line tool
+to interact with [Chef](https://chef.io), a platform to automate and manage infrastructure via
+code.
+
+To use it, add `knife` to the plugins array in your zshrc file:
+```zsh
+plugins=(... knife)
+```
+
+## Options
+
+- `KNIFE_RELATIVE_PATH`: if set to `true`, the completion script will look for local cookbooks
+  under the `cookbooks` folder in the chef root directory. It has preference over the other two
+  options below. **Default:** empty.
+
+- `KNIFE_COOKBOOK_PATH`: if set, it points to the folder that contains local cookbooks, for
+   example: `/path/to/my/chef/cookbooks`. **Default:** `cookbook_path` field in `knife.rb`
+   (see below).
+
+- `KNIFE_CONF_PATH`: variable pointing to the `knife.rb` configuration file, for example
+  `/path/to/my/.chef/knife.rb`. Only used if `$KNIFE_COOKBOOK_PATH` isn't set. If it exists,
+  `$PWD/.chef/knife.rb` is used instead. Otherwise, if it's set, its value is used.
+  **Default**: `$HOME/.chef/knife.rb`.