MDL-39092 JavaScript: Ignore all upstream YUI coverage JS

.gitignore

@@ -32,3 +32,4 @@ composer.lock
 /vendor/
 /behat.yml
 */yui/build/*/*-coverage.js
+/lib/yuilib/*/build/*/*-coverage.js

lib/yuilib/readme_moodle.txt

@@ -1,12 +1,13 @@
-Description of import of various YAHOO libraries into Moodle:
+Description of import of various YUI libraries into Moodle:
 
 1/ YUI2 version 2.9.0:
 * copy of 2in3 without older version
 * exact version specified in lib/setup.php
 
 2/ YUI3 version 3.9.1:
-* full copy of the "build" directory without any changes
+* full copy of the "build" directory. Unit test code coverage files are
+  removed but no other changes are made.
 * exact version specified in lib/setup.php
 
 Code downloaded from:
-http://developer.yahoo.com/yui
+http://yuilibrary.com