Add link to learn more about the issue which makes YoutubeStreamExtractorRelatedMixTest.testRelatedItems test disabled

Author: AudricV

extractor/src/test/java/org/schabi/newpipe/extractor/services/youtube/stream/YoutubeStreamExtractorRelatedMixTest.java

@@ -84,7 +84,7 @@ public static void setUp() throws Exception {
     // @formatter:on
 
     @Test
-    @Disabled("Mixes are not available in related items anymore")
+    @Disabled("Mixes are not available in related items anymore, see https://github.com/TeamNewPipe/NewPipeExtractor/issues/820")
     @Override
     public void testRelatedItems() throws Exception {
         super.testRelatedItems();