docs: clarify and include licenses of dependencies

While our contribution guide tries to make clear the licenses that apply
to libgit2, it does not make clear that different licenses apply to our
bundled dependencies. Make this clear by listing each dependency
together with the licenses that they are governed by. Furthermore,
bundle the complete license texts next to the code they apply to.

deps/zlib/COPYING

+Copyright (C) 1995-2017 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
+
+The data format used by the zlib library is described by RFCs
+(Request for Comments) 1950 to 1952 in the files rfc1950 (zlib
+format), rfc1951 (deflate format) and rfc1952 (gzip format).

docs/contributing.md

@@ -6,13 +6,22 @@ your help.
 ## Licensing
 
 By contributing to libgit2, you agree to release your contribution under
-the terms of the license.  Except for the `examples` directory, all code
-is released under the [GPL v2 with linking exception](COPYING).
+the terms of the license.  Except for the `examples` and the
+`deps` directories, all code is released under the [GPL v2 with
+linking exception](../COPYING).
 
 The `examples` code is governed by the
-[CC0 Public Domain Dedication](examples/COPYING), so that you may copy
+[CC0 Public Domain Dedication](../examples/COPYING), so that you may copy
 from them into your own application.
 
+The bundled dependencies in the `deps` directories are governed
+by the following licenses:
+
+- http-parser is licensed under [MIT license](../deps/http-parser/COPYING)
+- regex is governed by [LGPL v2.1+ license](../deps/regex/COPYING)
+- winhttp is governed by [LGPL v2.1+](../deps/winhttp/COPYING.LGPL) and [GPL v2 with linking exception](../deps/winhttp/COPYING.GPL)
+- zlib is governed by [zlib license](../deps/zlib/COPYING)
+
 ## Discussion & Chat
 
 We hang out in the