DOC: add GLFW copyright

Copyright/GLFWCopyright.txt

+Copyright (c) 2002-2006 Marcus Geelnard
+Copyright (c) 2006-2010 Camilla Berglund <elmindreda@elmindreda.org>
+
+This software is provided 'as-is', without any express or implied
+warranty. In no event will the authors be held liable for any damages
+arising from the use of this software.
+
+Permission is granted to anyone to use this software for any purpose,
+including commercial applications, and to alter it and redistribute it
+freely, subject to the following restrictions:
+
+1. The origin of this software must not be misrepresented; you must not
+   claim that you wrote the original software. If you use this software
+   in a product, an acknowledgment in the product documentation would
+   be appreciated but is not required.
+
+2. Altered source versions must be plainly marked as such, and must not
+   be misrepresented as being the original software.
+
+3. This notice may not be removed or altered from any source
+   distribution.
+