Fix BacktraceProcessor and improve README

Integration with Symfony and MonologBundle

You can register those processors to be used with Symfony and MonologBundle by adding the following configuration to
your config/packages/monolog.php file:

use Monolog\Processor\ProcessorInterface;

return static function (ContainerConfigurator $configurator): void {
    // ...

    // register as many processors as you like, but keep in mind that
    // each processor is called for each log record
    $services = $configurator->services();
    $services
        ->set(BacktraceProcessor::class)
        ->set(EnvVarProcessor::class)->args(['APP_ENV'])
        ->set(ProtocolVersionProcessor::class)
        ->set(SapiNameProcessor::class);

    // ...
};

If you don't use autoconfigure, you need to tag the processors with monolog.processor:

use Monolog\Processor\ProcessorInterface;
use MonologProcessorCollection\BacktraceProcessor;
use MonologProcessorCollection\EnvVarProcessor;

return static function (ContainerConfigurator $configurator): void {
    // ...

    $services = $configurator->services();
    $services
        ->set(BacktraceProcessorAlias::class)
            ->tag('monolog.processor', ['handler' => 'main'])
        ->set(EnvVarProcessor::class)->args(['APP_ENV'])
            ->tag('monolog.processor', ['handler' => 'main']);

    // ...
};

You can achieve the same configuration with YAML:

# config/packages/monolog.yaml
services:
    Monolog\Processor\BacktraceProcessor:
        tags:
            - { name: monolog.processor, handler: main }
    Monolog\Processor\EnvVarProcessor:
        arguments:
            - APP_ENV
        tags:
            - { name: monolog.processor, handler: main }

Or XML:

<!-- config/packages/monolog.xml -->

<!-- ... -->

<service id="Monolog\Processor\BacktraceProcessor" public="false">
    <tag name="monolog.processor" handler="main" />
</service>
<service id="Monolog\Processor\EnvVarProcessor" public="false">
    <argument>APP_ENV</argument>
    <tag name="monolog.processor" handler="main" />
</service>