docs: documenting adding a new test to arkscript for new contributors

CONTRIBUTING.md

@@ -34,7 +34,7 @@ Code formatting as well as static analysis is handled through `pre-commit` and i
 
 ### Running the test suites
 
-Build the tests using `-DARK_TESTS=On`. It is **strongly recommended** to add `-DARK_COVERAGE=On` to generate coverage reports.
+Build the tests using `-DARK_TESTS=On`.
 
 Run the `unittests` target to:
 - run various C++ unit tests,
@@ -43,6 +43,28 @@ Run the `unittests` target to:
 - run formatting tests,
 - run language tests (`tests/unittests/resources/LangSuite/`)
 
+#### Adding new tests
+
+The easiest tests to add are snapshot tests, in `tests/unittests/resources/...Suite/`. They expect an input `.ark` file and a desired output file (`.expected` or `.json` depending on the suite).
+
+- **ASTSuite** tests the AST parsing, which is then dumped to JSON by the JsonCompiler
+- **BytecodeReaderSuite** tests the decoding of ArkScript bytecode files
+- **CompilerSuite** tests the IR generation and optimization
+- **DiagnosticsSuite** tests the generation of error messages
+  - **compileTime** for errors generated by the compiler
+  - **runtime** for errors generated by the virtual machine
+- **FormatterSuite** tests the code formatter. Input is badly formatted, output is the expected formatted code
+- **LangSuite** are unit tests written in ArkScript itself, testing the builtins and the language
+- **NameResolutionSuite** tests the resolution of names and packages with import directives
+- **OptimizerSuite** tests the AST optimizer. Nodes should be removed by this pass, we ensure they are not present in the final AST
+- **ParserSuite** tests the parser, along with parsing specific errors
+  - **failure** for the parsing failures
+  - **success** for the successful parsing, output is the pretty printed AST
+
+#### Coverage reports
+
+It is **recommended** to add `-DARK_COVERAGE=On` to generate coverage reports, this way you can check if your feature has sufficient test coverage instead of waiting on the CI.
+
 Generate coverage reports with `cmake --build build --target coverage`, it will generate HTML files under `build/coverage/` (main file is `index.html`).
 
 ## C++ coding guidelines