Document the .editorconfig rules the sources follow and how to verify them

README.md

@@ -77,6 +77,18 @@ dotnet build            # or dotnet run --project src/Minesweeper
 dotnet test             # 65 tests
 ```
 
+## Code style
+
+Everything here follows the checked-in `.editorconfig`: UTF-8 BOM and four space indents
+for C#, two spaces for project files, `System` usings first, no redundant `this.`, and
+`s_` / `_` prefixes for private static and instance fields. A checkout can be verified
+without changing it:
+
+```bash
+dotnet format --verify-no-changes                # exits non-zero if anything drifted
+dotnet format whitespace --verify-no-changes     # layout only
+```
+
 ## Standalone binaries
 
 `./publish.sh` produces a self-contained, trimmed, single-file binary for each runtime under