unlicense: add 3 popular projects using it

Author: waldyrious

_licenses/unlicense.txt

@@ -6,9 +6,13 @@ hidden: false
 
 description: A license with no conditions whatsoever which dedicates works to the public domain. Unlicensed works, modifications, and larger works may be distributed under different terms and without source code.
 
-
 how: Create a text file (typically named UNLICENSE or UNLICENSE.txt) in the root of your source code and copy the text of the license disclaimer into the file.
 
+using:
+  - youtube-dl: https://github.com/rg3/youtube-dl/blob/master/LICENSE
+  - stacktrace.js: https://github.com/stacktracejs/stacktrace.js/blob/master/LICENSE
+  - kakokune: https://github.com/mawww/kakoune/blob/master/UNLICENSE
+
 permissions:
   - private-use
   - commercial-use