C#: Add module README

This should clarify development workflow with the NuGet packages.

modules/mono/README.md

@@ -0,0 +1,45 @@
+# How to build and run
+
+1. Build Godot with the module enabled: `module_mono_enabled=yes`.
+2. After building Godot, use it to generate the C# glue code:
+   ```sh
+   <godot_binary> --generate-mono-glue ./modules/mono/glue
+   ```
+3. Build the C# solutions:
+   ```sh
+   ./modules/mono/build_scripts/build_assemblies.py --godot-output-dir ./bin
+   ```
+
+The paths specified in these examples assume the command is being run from
+the Godot source root.
+
+# How to deal with NuGet packages
+
+We distribute the API assemblies, our source generators, and our custom
+MSBuild project SDK as NuGet packages. This is all transparent to the user,
+but it can make things complicated during development.
+
+In order to use Godot with a development of those packages, we must create
+a local NuGet source where MSBuild can find them. This can be  done with
+the .NET CLI:
+
+```sh
+dotnet nuget add source ~/MyLocalNugetSource --name MyLocalNugetSource
+```
+
+The Godot NuGet packages must be added to that local source. Additionally,
+we must  make sure there are no other versions of the package in the NuGet
+cache, as MSBuild may pick one of those instead.
+
+In order to simplify this process, the `build_assemblies.py` script provides
+the following `--push-nupkgs-local` option:
+
+```sh
+./modules/mono/build_scripts/build_assemblies.py --godot-output-dir ./bin \
+    --push-nupkgs-local ~/MyLocalNugetSource
+```
+
+This option ensures the packages will be added to the specified local NuGet
+source and that conflicting versions of the package are removed from the
+NuGet cache. It's recommended to always use this option when building the
+C# solutions during development to avoid mistakes.