dash: add README (#8221)

plugins/dash/README.md

+# Dash plugin
+
+This plugin adds command line functionality for [Dash](https://kapeli.com/dash),
+an API Documentation Browser for macOS. This plugin requires Dash to be installed
+to work.
+
+To use it, add `dash` to the plugins array in your zshrc file:
+
+```zsh
+plugins=(... dash)
+```
+
+## Usage
+
+- Open and switch to the dash application.
+```
+dash
+```
+
+- Query for something in dash app: `dash query`
+```
+dash golang 
+```
+
+- You can optionally provide a keyword: `dash [keyword:]query`
+```
+dash python:tuple
+```