title: NuGet package - readme.txt being added to target project, instead of just being displayed
body: <p>I'm responsible for a library published in our company as a NuGet package, and I followed the "<a href="https://docs.microsoft.com/en-us/nuget/create-packages/creating-a-package#adding-a-readme-and-other-files" rel="nofollow noreferrer">Adding a readme and other files</a>" guide to show a readme.txt containing important changes when users install or update the package when using Visual Studio. This works - i.e. when you install the package the readme.txt is displayed. However when you install the package this readme.txt files is also being copied into the root directory of the target project. </p>
  
  <p>Does anyone know how I can stop this (i.e. show the readme.txt when install, but <em>don't</em> add it to the target project)? It's quite useful for users to see this file, but confusing if it gets added to the project.</p>
  
  <p>The .nuspec for the library like the below:</p>
  
  <pre><code>&lt;?xml version="1.0"?&gt;
  &lt;package &gt;
    &lt;metadata&gt;
      &lt;id&gt;$id$&lt;/id&gt;
      &lt;version&gt;$version$&lt;/version&gt;
      &lt;title&gt;$title$&lt;/title&gt;
      &lt;authors&gt;$author$&lt;/authors&gt;
      &lt;owners&gt;$author$&lt;/owners&gt;
      &lt;projectUrl&gt;http://example.com/blah/blah&lt;/projectUrl&gt;
      &lt;requireLicenseAcceptance&gt;false&lt;/requireLicenseAcceptance&gt;
      &lt;description&gt;$description$&lt;/description&gt;
    &lt;/metadata&gt;
    &lt;files&gt;
      &lt;file src="README.txt" target="" /&gt;
    &lt;/files&gt;
  &lt;/package&gt;
  </code></pre>
  
  <p>The packing + pushing is normally performed by a TeamCity build step, but if I build the pkg using the following command line then the same behaviour is present:</p>
  
  <pre><code>$ nuget pack MyLibrary.csproj
  $ nuget push MyLibrary-{version}.nupkg -source {internal-nuget-server}
  </code></pre>