title: add System.Security.Cryptography.Xml library to .net framework 4.6 project
body: <p>I try to add package via nuget but this error occurs;</p>
  
  <blockquote>
    <p>Could not install package 'System.Security.Cryptography.Xml 4.4.2'.
    You are trying to install this package into a project that targets
    '.NETFramework,Version=v4.6', but the package does not contain any
    assembly references or content files that are compatible with that
    framework. For more information, contact the package author.</p>
  </blockquote>
  
  <p>Ok message is clear but there is a code peace and I moved it to my project. How can I make it work ? it requires this library</p>
  
  <pre><code>public string SignXml(XmlDocument xmlDoc, X509Certificate2 uidCert)
              {
                  try
                  {
                      // Create a SignedXml object.
                      SignedXml signedXml = new SignedXml(xmlDoc);
  </code></pre>