Add a style guide for project organization

This closes #3104.

getting_started/workflow/project_setup/project_organization.rst

@@ -44,6 +44,23 @@ They can then use a separate folder to store built levels that use them.
     /characters/npcs/suzanne/suzanne.png
     /levels/riverdale/riverdale.scn
 
+Style guide
+-----------
+
+For consistency across projects, we recommend following these guidelines:
+
+- Use **snake_case** for folder and file names (with the exception of C#
+  scripts). This sidesteps case sensitivity issues that can crop up after
+  exporting a project on Windows. C# scripts are an exception to this rule,
+  as the convention is to name them after the class name which should be
+  in PascalCase.
+- Use **PascalCase** for node names, as this matches built-in node casing.
+- In general, keep third-party resources in a top-level ``addons/`` folder, even
+  if they aren't editor plugins. This makes it easier to track which files are
+  third-party. There are some exceptions to this rule; for instance, if you use
+  third-party game assets for a character, it makes more sense to include them
+  within the same folder as the character scenes and scripts.
+
 Importing
 ---------