Add Prerequisites (#542)

Author: kblok

README.md

@@ -19,6 +19,10 @@ Puppeteer Sharp is a .NET port of the official [Node.JS Puppeteer API](https://g
 * [StackOverflow](https://stackoverflow.com/search?q=puppeteer-sharp)
 * [Issues](https://github.com/kblok/puppeteer-sharp/issues?utf8=%E2%9C%93&q=is%3Aissue)
 
+# Prerequisites
+
+ * As Puppeteer-Sharp is a NetStandard 2.0 library, The minimum platform versions are .NET Framework 4.6.1 and .NET Core 2.0. [Read more](https://docs.microsoft.com/en-us/dotnet/standard/net-standard).
+ * The minimum Windows versions supporting the WebSocket library are Windows 8 and Windows Server 2012. [Read more](https://docs.microsoft.com/en-us/dotnet/api/system.net.websockets?redirectedfrom=MSDN&view=netframework-4.7.2).
 
 # Usage
 

docfx_project/index.md

@@ -2,6 +2,11 @@
 
 Puppeteer Sharp is a .NET port of the official [Node.JS Puppeteer API](https://github.com/GoogleChrome/puppeteer). 
 
+# Prerequisites
+
+ * As Puppeteer-Sharp is a NetStandard 2.0 library, The minimum platform versions are .NET Framework 4.6.1 and .NET Core 2.0. [Read more](https://docs.microsoft.com/en-us/dotnet/standard/net-standard).
+ * The minimum Windows versions supporting the WebSocket library are Windows 8 and Windows Server 2012. [Read more](https://docs.microsoft.com/en-us/dotnet/api/system.net.websockets?redirectedfrom=MSDN&view=netframework-4.7.2).
+
 # Usage
 
 ## Take screenshots