x-server pre-requisite (#1459)

* x-server pre-requisite

* change copy

Co-authored-by: Darío Kondratiuk <dariokondratiuk@gmail.com>

README.md

@@ -26,6 +26,7 @@ Puppeteer Sharp is a .NET port of the official [Node.JS Puppeteer API](https://g
  * 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). If you need to run Puppeteer-Sharp on Windows 7 you can use [System.Net.WebSockets.Client.Managed](https://www.nuget.org/packages/System.Net.WebSockets.Client.Managed/) through the [LaunchOptions.WebSocketFactory](https://www.puppeteersharp.com/api/PuppeteerSharp.LaunchOptions.html#PuppeteerSharp_LaunchOptions_WebSocketFactory) property.
  * If you have issues running Chrome on Linux, the Puppeteer repo has a [great troubleshooting guide](https://github.com/GoogleChrome/puppeteer/blob/master/docs/troubleshooting.md).
+ * X-server is required on Linux.
 
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