Add license.txt file for the DMD snap

The license texts in this file are taken from the corresponding license
texts provided in the DMD, druntime and phobos source repositories.  An
overview has been added to summarize the different components, with the
zlib license also included since libphobos statically links to zlib when
it is built.

Note that the DMD compiler license takes into account the announcement
of 7 April 2017 that the backend has been relicensed under the terms of
the Boost Licence (Version 1.0):
https://forum.dlang.org/thread/oc8acc$1ei9$1@digitalmars.com
dlang/dmd#6680

... and in particular, see:
https://forum.dlang.org/post/oc8bie$1gql$1@digitalmars.com

snap/license.txt

@@ -0,0 +1,70 @@
+Overview
+========
+
+The DMD compiler is released under the Boost Software License (Version 1.0),
+as are druntime, the Phobos standard library, and the ancillary D tools
+(rdmd, dustmite and ddemangle).  Phobos statically links in zlib, which is
+distributed under the terms of the zlib license.
+
+
+Full license texts
+==================
+
+-- DMD compiler, druntime, phobos, rdmd, ddemangle, dustmite -------------------
+
+Boost Software License - Version 1.0 - August 17th, 2003
+
+Permission is hereby granted, free of charge, to any person or organization
+obtaining a copy of the software and accompanying documentation covered by
+this license (the "Software") to use, reproduce, display, distribute,
+execute, and transmit the Software, and to prepare derivative works of the
+Software, and to permit third-parties to whom the Software is furnished to
+do so, all subject to the following:
+
+The copyright notices in the Software and this entire statement, including
+the above license grant, this restriction and the following disclaimer,
+must be included in all copies of the Software, in whole or in part, and
+all derivative works of the Software, unless such copies or derivative
+works are solely in the form of machine-executable object code generated by
+a source language processor.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE, TITLE AND NON-INFRINGEMENT. IN NO EVENT
+SHALL THE COPYRIGHT HOLDERS OR ANYONE DISTRIBUTING THE SOFTWARE BE LIABLE
+FOR ANY DAMAGES OR OTHER LIABILITY, WHETHER IN CONTRACT, TORT OR OTHERWISE,
+ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+DEALINGS IN THE SOFTWARE.
+
+
+-- zlib license ----------------------------------------------------------------
+
+Acknowledgments:
+
+  The deflate format used by zlib was defined by Phil Katz. The deflate
+  and zlib specifications were written by Peter Deutsch. Thanks to all the
+  people who reported problems and suggested various improvements in zlib;
+  they are too numerous to cite here.
+
+Copyright notice:
+
+  Copyright (C) 1995-1998 Jean-loup Gailly and Mark Adler
+
+  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.
+
+  Jean-loup Gailly        Mark Adler
+  jloup@gzip.org          madler@alumni.caltech.edu