docs: Swoole backend

docs/backends/swoole.md

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+<span style="float:right;"><a href="https://github.com/RubixML/ML/blob/master/src/Backends/Swoole.php">[source]</a></span>
+
+# Swoole
+[Swoole](https://swoole.com)/[OpenSwoole](https://openswoole.com/) is an async PHP extension that also supports multiprocessing.
+
+!!! tip
+    Swoole backend makes use of [Igbinary](https://www.php.net/manual/en/intro.igbinary.php) serializer. If you need to
+    optimize the memory usage (or getting out-of-memory errors) consider installing [Igbinary](https://www.php.net/manual/en/intro.igbinary.php).
+
+## Example
+
+No parameters are required. It's a drop-in replacement for the [Serial](backends/serial.md) backend.
+
+```php
+use Rubix\ML\Backends\Swoole;
+
+$backend = new Swoole();
+```