Create CONTRIBUTING.md

.github/CONTRIBUTING.md

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+Vibes:
+======
+Generally-useful extensions are great, custom extensions and themes just for one specific DIY site
+are welcome too. I (Shish) will probably only actively maintain and add features to the extensions
+which I personally use, but if you submit some code of your own I will try to keep it updated and
+compatible with any API changes that come along. If your code comes with unit tests, this type of
+maintenance is much more likely to be successful :)
+
+Testing:
+========
+Github Actions will be running three sets of automated tests, all of which you can run for yourself:
+
+- `./vendor/bin/php-cs-fixer fix` - keeping a single style for the whole project
+- `./vendor/bin/phpunit --config tests/phpunit.xml` - unit testing
+- `./vendor/bin/phpstan analyse --memory-limit 1G -c tests/phpstan.neon` - type checking
+
+The `main` branch is locked down so it can't accept pull requests that don't pass these
+
+Testing FAQs:
+=============
+
+## What the heck is "Method XX::YY() return type has no value type specified in iterable type array."?
+
+PHP arrays are very loosely defined - they can be lists or maps, with integer or string
+(or non-continuous integer) keys, with any type of object (or multiple types of object).
+This isn't great for type safety, so PHPStan is a bit stricter, and requires you to
+specify what type of array it is and what it contains. You can do this with PHPdoc comments,
+like:
+
+```php
+/**
+ * @param array<string, Cake> $cakes -- a mapping like ["sponge" => new Cake()]
+ * @return array<Ingredient> -- a list like [new Ingredient("flour"), new Ingredient("egg")]
+ */
+function get_ingredients(array $cakes, string $cake_name): array {
+    return $cakes[$cake_name]->ingredients;
+}
+```