Update README.textile

Add upgrade command to docs. I canceled the prompt once and went hunting for this so I could upgrade.

README.textile

@@ -72,6 +72,8 @@ To disable updates entirely, put this in your ~/.zshrc
 
 @DISABLE_AUTO_UPDATE=true@
 
+To upgrade directly from the command line, just run @upgrade_oh_my_zsh@
+
 h3. Uninstalling
 
 If you want to uninstall it, just run @uninstall_oh_my_zsh@ from the command line and it'll remove itself and revert you to bash (or your previous zsh config).