Add Note About Adult Sites

The data.json file is easy to add to, so users will be able to maintain their own forks. This is not ideal. Maybe there could be another repo with an adult data.json? That would avoid forks getting out of date.

README.md

@@ -114,6 +114,8 @@ Please look at the Wiki entry on
 [adding new sites](https://github.com/TheYahya/sherlock/wiki/Adding-Sites-To-Sherlock)
 to understand the issues.
 
+**NOTE**: Sherlock is not accepting adult sites in the standard list.
+
 ## Tests
 If you are contributing to Sherlock, then Thank You!