I also received this on many sites.
Wordfence gave me this this domain as the problem: designcontest dotcom Is it the same for you?
From the thread below:
"The readme.txt can't be executed. It probably flagged one of the credit links at the end of the file. The plugin doesn't have any malware."
From:
https://wordpress.org/support/topic/malware-reported-in-wp-super-cache-readmetxt?replies=4