re-adding missing restore command, more detail in failing c# godot builds.

getting_started/scripting/c_sharp/c_sharp_basics.rst

@@ -161,10 +161,20 @@ located in the project root:
     </Project>
 
 
+Whenever packages are added or modified, run nuget restore in the root of the
+project directory, to ensure that the nuget packages will be available for msbuild
+(the tool used by godot to build mono projects) to use, run::
+
+  $ msbuild /t:restore
+
+
 Debugging Failing Godot Builds
 ------------------------------
 
 If you find that your mono project is failing to build, you can debug more closely
 by running the following on the command line, in the root of your project::
 
-    $ msbuild
+  $ msbuild
+
+This prints detailed output on why the build failed. Note that as mentioned
+in the above section, you need to run msbuild /t:restore separately.