Added Third Party Notices document

THIRD-PARTY-NOTICES.TXT

@@ -0,0 +1,32 @@
+ImageSharp uses third-party libraries or other resources that may be
+distributed under licenses different than ImageSharp itself.
+
+In the event that we accidentally failed to list a required notice, please
+bring it to our attention by posting an issue.
+
+The attached notices are provided for information only.
+
+
+License notice for Zlib
+-----
+
+DeflateStream implementation adapted from SharpZipLib.
+Licensed under MIT.
+https://github.com/icsharpcode/SharpZipLib
+
+
+Crc32 and Adler32 SIMD implementation adapted from Chromium.
+Licensed under BSD 3-Clause "New" or "Revised" License.
+https://github.com/chromium/chromium
+
+
+License notice for Stream Read/Write Extensions
+-----
+
+Licensed to the .NET Foundation under one or more agreements.
+The .NET Foundation licenses this file to you under the MIT license.
+See the LICENSE file in the CoreFX project root for more information.
+
+https://github.com/dotnet/corefx/blob/17300169760c61a90cab8d913636c1058a30a8c1/LICENSE.TXT
+https://github.com/dotnet/corefx/blob/17300169760c61a90cab8d913636c1058a30a8c1/src/Common/src/CoreLib/System/IO/Stream.cs#L742
+https://github.com/dotnet/corefx/blob/17300169760c61a90cab8d913636c1058a30a8c1/src/Common/src/CoreLib/System/IO/Stream.cs#L775