[css-writing-modes-3] Update implementation-report introduction with Alan's advice

Author: kojiishi

css-writing-modes-3/implementation-report.html

@@ -44,7 +44,12 @@ <h2>Introduction</h2>
     (e.g. Hebrew or Arabic), bidirectional (e.g. mixed Latin and Arabic) and vertical (e.g. Asian scripts).
     It has a <a href="http://test.csswg.org/suites/css-writing-modes-3_dev/nightly-unstable/">test suite</a>
     containing 995 tests. Of those, 866 (87%) meet the CR exit criteria as of January, 2017.</p>
-  <p>This implementation report examines the remaining tests in more detail.
+  <p>
+    This implementation report examines the remaining tests in more detail.
+    If one filters out tests for the next level of Writing Modes,
+    tests for non-MUST requirements,
+    and tests that require other specifications (all listed in detail below)
+    the test suite contains 879 tests with 865 (98%) meeting the CR exit criteria.
   </p>
   <p>Note that this report counts Blink and WebKit as one implementation,
     though it shows the data for each where available.</p>