update readme

README.md

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-sugar
+Sugar
 -----
 
 Functions that improve the readability of Racket code.
 
+Using Racket 6.0+, install from the command line like so:
+
+    raco pkg install sugar
+
+And update like so:
+
+    raco pkg update sugar
+
 You can [read the docs here](http://mbutterick.github.io/sugar/doc/).