README: Expand the at-end-of-.zshrc FAQ answer.

Fixes zsh-users/zsh-syntax-highlighting#185.

README.md

@@ -54,7 +54,9 @@ FAQ
 ### Why must `zsh-syntax-highlighting.zsh` be sourced at the end of the `.zshrc` file?
 
 `zsh-syntax-highlighting.zsh` wraps ZLE widgets.  It must be sourced after all
-custom widgets have been created (i.e., after all `zle -N` calls).
+custom widgets have been created (i.e., after all `zle -N` calls and after
+running `compinit`).  Widgets created later will work, but will not update the
+syntax highlighting.
 
 How to tweak
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