Add preparation instructions to Windows build script

Signed-off-by: Aanand Prasad <aanand.prasad@gmail.com>

script/build-windows.ps1

@@ -1,3 +1,33 @@
+# Builds the Windows binary.
+#
+# From a fresh 64-bit Windows 10 install, prepare the system as follows:
+#
+# 1. Install Git:
+#
+#        http://git-scm.com/download/win
+#
+# 2. Install Python 2.7.10:
+#
+#        https://www.python.org/downloads/
+#
+# 3. Append ";C:\Python27;C:\Python27\Scripts" to the "Path" environment variable:
+#
+#        https://www.microsoft.com/resources/documentation/windows/xp/all/proddocs/en-us/sysdm_advancd_environmnt_addchange_variable.mspx?mfr=true
+#
+# 4. In Powershell, run the following commands:
+#
+#        $ pip install virtualenv
+#        $ Set-ExecutionPolicy -Scope CurrentUser RemoteSigned
+#
+# 5. Clone the repository:
+#
+#        $ git clone https://github.com/docker/compose.git
+#        $ cd compose
+#
+# 6. Build the binary:
+#
+#        .\script\build-windows.ps1
+
 $ErrorActionPreference = "Stop"
 Set-PSDebug -trace 1