docs: add README.md for Dash plugin

plugins/dash/README.md

+# Dash plugin
+
+This plugin adds command line functionality for [Dash](https://kapeli.com/dash),
+an API Documentation Browser.
+
+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)
+```
+
+Make sure to source your rc file:
+```
+source ~/.zshrc
+```
+
+## Usage
+
+Open and switch to the dash application.
+```
+dash
+```
+
+Query for something in dash app.
+```
+dash golang
+```