Improve readme, document improved ReflectionDescriptor

Author: janedbal

README.md

@@ -128,6 +128,12 @@ Queries are analyzed statically and do not require a running database server. Th
 
 Most DQL features are supported, including `GROUP BY`, `DISTINCT`, all flavors of `JOIN`, arithmetic expressions, functions, aggregations, `NEW`, etc. Sub queries and `INDEX BY` are not yet supported (infered type will be `mixed`).
 
+### Query type inference of expressions
+
+Whether `MAX(e.id)` is fetched as `numeric-string` or `int` highly [depends on drivers, their setup and PHP version](https://github.com/janedbal/php-database-drivers-fetch-test).
+This extension autodetects your setup and provides accurate results for `pdo_mysql`, `mysqli`, `pdo_sqlite`, `sqlite3`, `pdo_pgsql` and `pgsql`.
+Any other driver will result in mixed.
+
 ### Supported methods
 
 The `getResult` method is supported when called without argument, or with the hydrateMode argument set to `Query::HYDRATE_OBJECT`:
@@ -152,12 +158,6 @@ $query->getOneOrNullResult(Query::HYDRATE_OBJECT); // User
 
 This is due to the design of the `Query` class preventing from determining the hydration mode used by these functions unless it is specified explicitly during the call.
 
-### Expression types inferring
-
-Whether `MAX(e.id)` is fetched as `numeric-string` or `int` highly [depends on drivers, their setup and PHP version](https://github.com/janedbal/php-database-drivers-fetch-test).
-This extension autodetects your setup and provides accurate results for `pdo_mysql`, `mysqli`, `pdo_sqlite`, `sqlite3`, `pdo_pgsql` and `pgsql`.
-Any other driver will result in mixed.
-
 ### Problematic approaches
 
 Not every QueryBuilder can be statically analysed, here are few advices to maximize type inferring:
@@ -201,6 +201,10 @@ Type descriptors don't have to deal with nullable types, as these are transparen
 
 If your custom type's `convertToPHPValue()` and `convertToDatabaseValue()` methods have proper typehints, you don't have to write your own descriptor for it. The `PHPStan\Type\Doctrine\Descriptors\ReflectionDescriptor` can analyse the typehints and do the rest for you.
 
+If parent of your type is one of the Doctrine's non-abstract ones, `ReflectionDescriptor` will reuse its descriptor even for expression resolution (e.g. `AVG(t.cost)`).
+For example, if you extend `Doctrine\DBAL\Types\DecimalType`, it will know that sqlite fetches that as `float|int` and other drivers as `numeric-string`.
+If you extend only `Doctrine\DBAL\Types\Type`, you should use custom descriptor and optionally implement even `DoctrineTypeDriverAwareDescriptor` to provide driver-specific resolution.
+
 ### Registering type descriptors
 
 When you write a custom type descriptor, you have to let PHPStan know about it. Add something like this into your `phpstan.neon`: