Add instructions for adding new syntax support

Author: nikic

CONTRIBUTING.md

@@ -2,3 +2,31 @@
 
 This project uses PSR-12 with consistent brace placement. This means that the opening brace is
 always on the same line, even for class and method declarations.
+
+## Tools
+
+This project uses PHP-CS-Fixer and PHPStan. You can invoke them using `make`:
+
+```shell
+make php-cs-fixer
+make phpstan
+```
+
+## Adding support for new PHP syntax
+
+1. If necessary, add emulation support for new tokens.
+   * Add a new subclass of `Lexer\TokenEmulator`. Take inspiration from existing classes.
+   * Add the new class to the array in `Lexer\Emulative`.
+   * Add tests for the emulation in `Lexer\EmulativeTest`. You'll want to modify
+     `provideTestReplaceKeywords()` for new reserved keywords and `provideTestLexNewFeatures()` for
+     other emulations.
+2. Add any new node classes that are needed.
+3. Add support for the new syntax in `grammar/php.y`. Regenerate the parser by running
+   `php grammar/rebuildParsers.php`. Use `--debug` if there are conflicts.
+4. Add pretty-printing support by implementing a `pFooBar()` method in `PrettyPrinter\Standard`.
+5. Add tests both in `test/code/parser` and `test/code/prettyPrinter`.
+6. Add support for formatting-preserving pretty-printing. This is done by modifying the data tables
+   at the end of `PrettyPrinterAbstract`. Add a test in `test/code/formatPreservation`.
+7. Does the new syntax feature namespaced names? If so, add support for name resolution in
+   `NodeVisitor\NameResolver`. Test it in `NodeVisitor\NameResolverTest`.
+8. Does the new syntax require any changes to builders? Is so, make them :)