Restore Drake's IWYU exception into the newly-created section on IWYU

Author: jwnimmer-tri

cppguide.html

@@ -288,6 +288,16 @@ <h3 id="Include_What_You_Use">Include What You Use</h3>
 symbol from it even if <code>foo.h</code>
 includes <code>bar.h</code>.</p>
 
+<p class="drake"><a name="iwyu_exception"/>
+While we encourage you to Include What You Use (IWYU) since it improves code
+transparency and readability, it will not be strictly enforced. Instead, we
+enforce a “weak include” style that simply requires every symbol referenced
+within a file be covered by the transitive closure of all #include
+statements in the file. We decided to make this exception because (1) we can
+always adopt an IWYU rule later, (2) to reduce verbosity, and (3) we don’t
+have a tool to enforce IWYU at this time.
+</p>
+
 <h3 id="Forward_Declarations">Forward Declarations</h3>
 
 <p>Avoid using forward declarations where possible.