Warn about issues with the Common Crawl corpus data (#3)

- Add warning to dataset card about corrupted source data (3f99bd92914750450d53148f0c88ba2cb664c6ec)
- Rephrase as data issues (03ad3a6938d7a32fdd830f27396702bc7ad40e84)

README.md

@@ -188,6 +188,15 @@ dataset_info:
 
 ### Dataset Summary
 
+<div class="course-tip course-tip-orange bg-gradient-to-br dark:bg-gradient-to-r before:border-orange-500 dark:before:border-orange-800 from-orange-50 dark:from-gray-900 to-white dark:to-gray-950 border border-orange-50 text-orange-700 dark:text-gray-400">
+  <p><b>Warning:</b> There are issues with the Common Crawl corpus data (<a href="https://www.statmt.org/wmt13/training-parallel-commoncrawl.tgz">training-parallel-commoncrawl.tgz</a>):</p>
+  <ul>
+    <li>Non-English files contain many English sentences.</li>
+    <li>Their "parallel" sentences in English are not aligned: they are uncorrelated with their counterpart.</li>
+  </ul>
+  <p>We have contacted the WMT organizers.</p>
+</div>
+
 Translation dataset based on the data from statmt.org.
 
 Versions exist for different years using a combination of data