title: Open-source: licence header on each source file OR a single COPYING OR both?
body: <p>Publishing an open-source project, is it enough to add a <strong>COPYING</strong> file to the package or do I need to copy and paste it on top of every project's source file?<br>
  On famous public repositories like <a href="http://github.com" rel="noreferrer">github</a> or Google code I've seen different mixed approaches so I would like to know how this aspect should be handled correctly.</p>