README: Add statement about web engine based help viewer

Even though it is stating an obvious prerequisite, it might help
people to realize that the web engine based backend exists.

Change-Id: I8d43d8a0fd7786de050b67d1438fefa3b95ec7df
Reviewed-by: hjk <hjk@qt.io>
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>

README.md

@@ -15,6 +15,7 @@ macOS 10.8 or later
 Prerequisites:
 
 * Qt 5.6.0 or later
+* Qt WebEngine module for QtWebEngine based help viewer
 * On Windows:
     * ActiveState Active Perl
     * MinGW with g++ 4.8 or Visual Studio 2015 or later