document the updated qmake/rules interaction

build-pcre.txt

@@ -1,3 +1,22 @@
+PCRE is a library that is used by the optional "rules" feature. (It adds
+some additional features to the command line client.) It's readily available
+on Linux and Mac OS X, but must be obtained separately for Windows.
+
+If you're using qmake to generate makefiles, the following behavior applies:
+
+- If you're not on Windows, it assumes by default that you have PCRE and want
+    to enable rules support.
+
+- If you're not on Windows, you can disable rules support (removing the PCRE
+    dependency) by passing HAVE_RULES=no to qmake.
+
+- If you are on Windows, but have PCRE available, you can enable rules support
+    by passing HAVE_RULES=yes to qmake.
+
+    - Note: This includes using build.bat since it calls qmake - to use PCRE and
+        build.bat, you need to edit build.bat to add HAVE_RULES=yes to the
+        qmake call.
+
 
 Some temporary build instructions. This is work in progress.