Adding line about switching to zsh.

README.md

@@ -20,6 +20,10 @@ From your cloned directory:
 This will create symlinks for all config files in your home directory. You can
 safely run this file multiple times to update.
 
+Note that there is configuration for `zsh` so if you'd like to switch your shell from the default `bash` to `zsh`, on OS X you do:
+
+    chsh -s /bin/zsh
+
 Track thoughtbot/dotfiles
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