Add copyright notice to sympa.js

Created by: xavierba

sympa.js is missing a copyright notice. I traced back the code to 2010 : https://github.com/sympa-community/sympa/commits/8af6368fe8acec2caa0c66298e6614eed969d6b3/src/etc/script/js/sympa.js @dverdin and @ikedas are the ones who significantly touched the code. I chose to use a short copyright notice rather than the full GPL boilerplate in order to not add too much to the transfer size of the javascript to the client. @dverdin , @ikedas, do you both agree to the licensing (same as sympa itself) and the shape of the copyright notice ?