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_UNIX.md

@@ -19,9 +19,10 @@ You need to install the following software to build SoftEther VPN for UNIX.
 
 For example, the following commands help you to install the above programs
 on Fedora or CentOS Linux:
-
+```
 $ yum -y groupinstall "Development Tools"
 $ yum -y install readline-devel ncurses-devel openssl-devel
+```
 
 
 How to Build
@@ -29,8 +30,10 @@ How to Build
 
 To build the programs from the source code, run the following commands:
 
+```
 $ ./configure
 $ make
+```
 
 If any error occurs, please check the above requirements.
 
@@ -41,7 +44,9 @@ How to Install SoftEther VPN Server, Bridge or Client
 To install the vpnserver, vpnbridge and vpnclient programs into the
 /usr/bin directory, run the following as the root user:
 
+```
 # make install
+```
 
 After the installation will complete successfully:
 
@@ -61,16 +66,22 @@ How to Run SoftEther VPN Server for Test
 
 To start the SoftEther VPN Server background service, run the following:
 
+```
 $ bin/vpnserver/vpnserver start
+```
 
 To stop the service, run the following:
 
+```
 $ bin/vpnserver/vpnserver stop
+```
 
 To configure the running SoftEther VPN Server service,
 you can use SoftEther VPN Command Line Management Utility as following:
 
+```
 $ bin/vpncmd/vpncmd
+```
 
 Or you can also use VPN Server Manager GUI Tool on other Windows PC to
 connect to the VPN Server remotely. You can download the GUI Tool
@@ -82,16 +93,22 @@ How to Run SoftEther VPN Bridge for Test
 
 To start the SoftEther VPN Bridge background service, run the following:
 
+```
 $ bin/vpnbridge/vpnbridge start
+```
 
 To stop the service, run the following:
 
+```
 $ bin/vpnbridge/vpnbridge stop
+```
 
 To configure the running SoftEther VPN Bridge service,
 you can use SoftEther VPN Command Line Management Utility as following:
 
+```
 $ bin/vpncmd/vpncmd
+```
 
 Or you can also use VPN Server Manager GUI Tool on other Windows PC to
 connect to the VPN Bridge remotely. You can download the GUI Tool
@@ -103,16 +120,22 @@ How to Run SoftEther VPN Client for Test
 
 To start the SoftEther VPN Client background service, run the following:
 
+```
 $ bin/vpnclient/vpnclient start
+```
 
 To stop the service, run the following:
 
+```
 $ bin/vpnclient/vpnclient stop
+```
 
 To configure the running SoftEther VPN Client service,
 you can use SoftEther VPN Command Line Management Utility as following:
 
+```
 $ bin/vpncmd/vpncmd
+```
 
 Or you can also use VPN Client Manager GUI Tool on other Windows PC to
 connect to the VPN Client remotely. You can download the GUI Tool