Update requirements note (#2509)

Author: chawyehsu

README.md

@@ -4,7 +4,8 @@ Scoop is a command-line installer for Windows.
 
 ## Requirements
 
-* [PowerShell 3](https://www.microsoft.com/en-us/download/details.aspx?id=34595) (or later)
+* Windows 7 SP1+ / Windows Server 2008+
+* [PowerShell 3](https://www.microsoft.com/en-us/download/details.aspx?id=34595) (or later) and [.NET Framework 4.5+](https://www.microsoft.com/net/download)
 * PowerShell must be enabled for your user account e.g. `set-executionpolicy remotesigned -s currentuser`
 
 ## Installation