Add zsh tab completion info

Ref: eddiezane/lunchy#57 (comment)

README.md

@@ -105,6 +105,14 @@ source /FULL/PATH/TO/wp-completion.bash
 
 Don't forget to run `source ~/.bash_profile` afterwards.
 
+If using zsh for your shell, you may need to do the following:
+
+```
+autoload bashcompinit
+bashcompinit
+source /FULL/PATH/TO/wp-completion.bash
+```
+
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