update docs with min Windows version and dev dependency on VS2015. (microsoft#228)

Author: Stan Iliev

docs/contribute.md

@@ -6,6 +6,8 @@
 
 ![Win](res/win_sm.png) Git for Windows [Install Here](https://git-scm.com/downloads) _(needed for dev sh script)
 
+![Win](res/win_sm.png) On Windows you need Visual Studio 2015.
+
 ## Build, Test, Clean, Restore 
 
 From src:

docs/preview/latebreaking.md

@@ -2,9 +2,9 @@
 
 ## Windows
 
-Preview 1 on tested on Windows 10 so far.
+The minimum supported version is Windows 7.
 
-Even though the agent has no pre-requisites, many of the tasks we run require Visual Studio 2015
+Even though the agent has no pre-requisites, many of the tasks we run require Visual Studio.
 
 ## ![osx](../res/apple_med.png) OSX