mediawiki.mixins.less: Add a link to source of the .background-image-svg technique

http://pauginer.tumblr.com/post/36614680636/invisible-gradient-technique

Change-Id: I2418f7704fef31d3a3bee8eed5e8bfb628086df3
Merge "mediawiki.mixins.less: Add a link to source of the .background-image-svg technique"