Added instructions to the developer docs for running the project.

docs/developers/Contributing.md

@@ -10,6 +10,15 @@
 There are also some [Item Templates](https://github.com/AvaloniaUI/avalonia-dotnet-templates) for Avalonia that can be very useful to have,
 helping you make things like new 'Windows' and 'Controls' easier.
 
+### Running the project
+
+- Make a fork of the project and clone it
+- Go to `NexusMods.App/src/NexusMods.App` and run `dotnet run` to run the executable file `NexusMods.App.csproj`
+  - The app should open and run if you are using JetBrains Rider
+  - If you are using Visual Studio, you may come across build error CS0579 in which you can try running `git submodule update --init --recursive` to update the SMAPI API
+    - If the command doesn't work then you can alternatively get the project running by cloning in either Git Bash or Command Prompt and using the same above commands (first update the submodule, then run the executable)
+
+
 ### Writing code
 
 Make sure to follow our [Development Guidelines](./development-guidelines/UICodingGuidelines.md).