Add note in README about how to workaround installation with existing .zshrc

README.md

@@ -27,6 +27,9 @@ version is 4.3.17.
           ln -s "$rcfile" "${ZDOTDIR:-$HOME}/.${rcfile:t}"
         done
 
+    NB: The above will not overwrite or update any existing `.zshrc`. Therefore, if you
+    have an existing `.zshrc` you must manually add the line `source "${ZDOTDIR:-$HOME}/.zprezto/init.zsh"`.
+
   4. Set Zsh as your default shell:
 
         chsh -s /bin/zsh