Add explicit warning that Coq 8.4 or later is required

src/Intro.v

@@ -187,7 +187,7 @@ Some readers have asked about the pragmatics of using this tactic library in the
 (** ** Installation and Emacs Set-Up *)
 
 (**
-At the start of the next chapter, I assume that you have installed Coq and Proof General.  The code in this book is tested with Coq version 8.4, though parts may work with other versions.
+At the start of the next chapter, I assume that you have installed Coq and Proof General.  The code in this book is tested with Coq versions 8.4 and 8.4pl1.  Though parts may work with other versions, it is expected that the book source will fail to build with _earlier_ versions.
 
 %\index{Proof General|(}%To set up your Proof General environment to process the source to this chapter, a few simple steps are required.