Add NuGet as a requirement for Mono compiling

Compiling Godot Mono on x11 with the latest source brings needs NuGet to be intall which is not a dependency of any current requirement.

development/compiling/compiling_with_mono.rst

@@ -11,6 +11,7 @@ Requirements
 - Mono 5.12.0 or greater
 - MSBuild
 - pkg-config
+- NuGet
 
 Environment variables
 ---------------------