Add a readme file with the thirdparty dll sources.

Closes #6863.

thirdparty/README

@@ -0,0 +1,44 @@
+The DLLs in this directory are sourced from:
+
+Eluant.dll
+	https://github.com/pchote/Eluant/
+	Commit 483f326bd071024f58dc7917da81e31ef680e85e
+	MIT license
+
+FuzzyLogicLibrary.dll
+	http://sourceforge.net/projects/fuzzynet/
+	Version 1.2.0
+	GPL v2.0 license
+
+ICSharpCode.SharpZipLib.dll
+	http://icsharpcode.github.io/SharpZipLib/
+	Version 0.86
+	GPL 2.0 with GNU Classpath exception
+
+MaxMind.Db.dll
+MaxMind.GeoIP2.dll
+Newtonsoft.Json.dll
+RestSharp.dll
+	https://github.com/maxmind/GeoIP2-dotnet/
+	Version 2.1.0
+	Apache 2.0 license
+
+Mono.Nat.dll
+	http://projects.qnetp.net/projects/show/mono-nat
+	Version 1.1.0
+	MIT license
+
+nunit.framework.dll
+	http://www.nunit.org/index.php?p=download
+	Version 2.6.3
+	zlib license
+
+SDL2-CS.dll
+	https://github.com/flibitijibibo/SDL2-CS/
+	Commit e8731efadb249a40f46d646e2b783ec588326ea6
+	zlib license
+
+SharpFont.dll
+	https://github.com/Robmaister/SharpFont
+	Version 2.5.0.1
+	MIT license