PHP session extension required.

At https://github.com/flarum/docs/blob/master/docs/install.md we find the list of PHP extensions that must be installed.

It is missing a reference to `session`. As a result, the user may run into the following error:
```
+ php flarum migrate
PHP Fatal error: Uncaught Error: Interface "SessionHandlerInterface" not found in /usr/local/www/chat.spectre.app/flarum/vendor/illuminate/session/FileSessionHandler.php:
10
Stack trace:
#0 /usr/local/www/chat.spectre.app/flarum/vendor/composer/ClassLoader.php(576): include()
#1 /usr/local/www/chat.spectre.app/flarum/vendor/composer/ClassLoader.php(427): Composer\Autoload\{closure}()
#2 /usr/local/www/chat.spectre.app/flarum/vendor/illuminate/session/SessionManager.php(73): Composer\Autoload\ClassLoader->loadClass()
#3 /usr/local/www/chat.spectre.app/flarum/vendor/illuminate/session/SessionManager.php(61): Illuminate\Session\SessionManager->createNativeDriver()
#4 /usr/local/www/chat.spectre.app/flarum/vendor/illuminate/support/Manager.php(105): Illuminate\Session\SessionManager->createFileDriver()
#5 /usr/local/www/chat.spectre.app/flarum/vendor/illuminate/support/Manager.php(80): Illuminate\Support\Manager->createDriver()
#6 /usr/local/www/chat.spectre.app/flarum/vendor/flarum/core/src/User/SessionManager.php(33): Illuminate\Support\Manager->driver()
```