docs: add how to update PHPStan baseline

Author: kenjis

contributing/pull_request.md

@@ -210,12 +210,28 @@ These tools have already been integrated into our CI/CD workflow to minimize una
 are expected that their code will pass these two. In your local machine, you can manually run these tools
 so that you can fix whatever errors that pop up with your submission.
 
+#### PHPStan
+
 PHPStan is expected to scan the entire framework by running this command in your terminal:
 
 ```console
 vendor/bin/phpstan analyse
 ```
 
+See also:
+- [PHPDocs Basics](https://phpstan.org/writing-php-code/phpdocs-basics)
+- [PHPDoc Types](https://phpstan.org/writing-php-code/phpdoc-types)
+
+If PHPStan errors cannot be resolved by any means, or if the PHPStan error is
+false positive and should be ignored, the baseline can be updated with the following
+command:
+
+```console
+vendor/bin/phpstan analyze --generate-baseline phpstan-baseline.php
+```
+
+#### Rector
+
 Rector, on the other hand, can be run on the specific files you modified or added:
 
 ```console