Added supported versions to the README

Author: thinkingserious

CONTRIBUTING.md

@@ -64,6 +64,7 @@ We welcome direct contributions to the csharp-http-client code base. Thank you!
 
 ##### Prerequisites #####
 
+- .NET version 4.5.2
 - Microsoft Visual Studio Community 2015 or greater
 
 ##### Initial setup: #####

README.md

@@ -10,6 +10,12 @@ All updates to this project is documented in our [CHANGELOG](https://github.com/
 
 # Installation
 
+## Prerequisites
+
+- .NET version 4.5.2
+
+## Install Package
+
 To use CSharp.HTTP.Client in your C# project, you can either <a href="https://github.com/sendgrid/csharp-http-client.git">download the SendGrid C# .NET libraries directly from our Github repository</a> or, if you have the NuGet package manager installed, you can grab them automatically.
 
 ```