adding icon in the nuspec file

Author: david-schrenker

manifest/icon.png

@@ -7,7 +7,7 @@
         <authors>Joe Cooper, David Schrenker, Matt Reibach, Ryan Lydzinski (Contributor)</authors>
         <license type="file">license.txt</license>
         <projectUrl>https://github.com/SocketLabs/socketlabs-csharp</projectUrl>
-        <iconUrl>http://static.socketlabs.com/logos/icon-512x512.png</iconUrl>
+        <icon type="file">icon.png</iconUrl>
         <requireLicenseAcceptance>false</requireLicenseAcceptance>
         <description>SocketLabs .Net Client Library.</description>
         <summary>Email Delivery Library for sending transactional and bulk email through the SocketLabs Platform.</summary>
@@ -32,5 +32,6 @@
         <file src="..\src\SocketLabs\bin\Release\netstandard2.0\SocketLabs.xml" target="lib\netstandard2.0\SocketLabs.xml" />
         <file src="..\manifest\license.txt" target="" />
         <file src="..\manifest\readme.txt" target="" />
+        <file src="..\manifest\icon.png" target="" />
     </files>
 </package>