Math.NET Numerics: 4.9.1 docs update

Build.html

@@ -61,6 +61,10 @@
 <p>If you do not want to use the official binaries, or if you like to modify,
 debug or contribute, you can compile locally either using Visual Studio or
 manually with the build scripts.</p>
+<h2><a name="System-Requirements" class="anchor" href="#System-Requirements">System Requirements</a></h2>
+<ul>
+<li>.NET Core SDK 3.1.1 (<a href="https://dotnet.microsoft.com/download/dotnet-core/3.1">download</a>)</li>
+</ul>
 <h2><a name="VisualStudio-or-Xamarin-Studio" class="anchor" href="#VisualStudio-or-Xamarin-Studio">VisualStudio or Xamarin Studio</a></h2>
 <p>We clearly separate dependency management from the IDE, you should therefore
 run <code>restore.cmd</code> or <code>restore.sh</code> once after every git checkout in order to restore