Add Unity project path to Unity example in docs

For Ale to recognize the symbols in your project, you need to point it
at where Unity builds your assemblies. Add this path to the example.

doc/ale-cs.txt

@@ -94,6 +94,7 @@ g:ale_cs_mcsc_assemblies                             *g:ale_cs_mcsc_assemblies*
     " Compile C# programs with the Unity engine DLL file on Mac.
     let g:ale_cs_mcsc_assemblies = [
     \ '/Applications/Unity/Unity.app/Contents/Frameworks/Managed/UnityEngine.dll',
+    \ 'path-to-unityproject/obj/Debug',
     \]
 <