COPYING: clarify that Daniel is not the sole author

... done on request and as it is a fair point.

COPYING: clarify that Daniel is not the sole author
86d3d62f · Daniel Stenberg · a6208704 · 86d3d62f
Commit 86d3d62f authored 9 years ago by Daniel Stenberg
--- a/COPYING
+++ b/COPYING
 COPYRIGHT AND PERMISSION NOTICE

-Copyright (c) 1996 - 2016, Daniel Stenberg, <daniel@haxx.se>.
+Copyright (c) 1996 - 2016, Daniel Stenberg, <daniel@haxx.se>, and many
+contributors, see the THANKS file.

 All rights reserved.