Remove redundant section and document keyword args

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.
+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:
 
@@ -11,19 +10,19 @@ To use it, add `dash` to the plugins array in your zshrc file:
 plugins=(... dash)
 ```
 
-Make sure to source your rc file:
-```
-source ~/.zshrc
-```
-
 ## Usage
 
-Open and switch to the dash application.
+- Open and switch to the dash application.
 ```
 dash
 ```
 
-Query for something in dash app.
+- Query for something in dash app: `dash query`
+```
+dash golang 
+```
+
+- You can optionally provide a keyword: `dash [keyword:]query`
 ```
-dash golang
+dash python:tuple
 ```