build documentation refactoring (#395)

BUILD_WINDOWS.TXT, BUILD_UNIX.TXT removed from root folder

src/BUILD_WINDOWS.TXT, src/BUILD_UNIX.TXT converted to "md"

BUILD_WINDOWS.TXT, BUILD_UNIX.TXT are now hyperlinks in README.md

src/BUILD_WINDOWS.md

@@ -0,0 +1,44 @@
+How to build SoftEther VPN for Windows
+======================================
+
+
+Requirements
+------------
+
+You need to install the following software to build SoftEther VPN for Windows.
+
+- Microsoft Windows XP, Vista, 7, 8 or later.
+- Microsoft Visual Studio 2008 with the latest SP (SP1 9.0.30729.4462 QFE).
+  Make sure that you installed the x64 compiler and build tools.
+
+* Note:
+  Visual Studio 2008 SP1 is required to build SoftEther VPN on Windows.
+  Please make sure that VS2008 'SP1' is installed.
+  Visual Studio 2010, 2012 or 2013 is currently not supported.
+  Visual Studio 2008 Express Edition is not supported.
+  Standard Edition, Professional Edition, Team System or Team Suite is
+  required.
+
+
+Full Build Instructions
+-----------------------
+
+The following steps will build all SoftEther VPN program files, and also build
+the installer packages of SoftEther VPN. It is very easy.
+
+1. Run the "BuildAll.cmd" batch file in the "src" directory.
+2. Wait until the building process will complete.
+3. The built files are stored on the "output" directory.
+
+
+Partly Build, Debug or Development Instructions on Visual Studio 2008
+---------------------------------------------------------------------
+
+If you are a programmer, you can open the SoftEther VPN solution file
+with Visual Studio 2008 to customize. Open "src\SEVPN.sln" and enjoy it.
+
+
+************************************
+Thank You Using SoftEther VPN !
+By SoftEther VPN Open-Source Project
+http://www.softether.org/