docfx_getting_started: Mention mono-devel

The primary aim here is to ensure that `mono-devel` is mentioned, since it's not enough in this case to just have the runtime package installed.

Closes dotnet#5082.

tutorial/docfx_getting_started.html

@@ -116,7 +116,7 @@ <h2 id="2-use-docfx-as-a-command-line-tool">2. Use <em>DocFX</em> as a command-l
 <p>Now you can view the generated website on http://localhost:8080.</p>
 <div class="IMPORTANT">
 <h5>Important</h5>
-<p>For macOS/Linux users: Use <a href="https://www.mono-project.com/">Mono</a> version &gt;= 5.10.</p>
+<p>For macOS/Linux users: Use <a href="https://www.mono-project.com/">Mono</a> version &gt;= 5.10. The following command line can be used on Debian/Ubuntu: <tt>sudo apt-get install mono-runtime mono-devel</tt></p>
 <p>For macOS users: <strong>DO NOT</strong> Install Mono from Homebrew, but rather from the <a href="https://www.mono-project.com/download/stable/#download-mac">Mono download page</a>.</p>
 </div>
 <h2 id="3-use-docfx-integrated-with-visual-studio">3. Use <em>DocFX</em> integrated with Visual Studio</h2>