Update README.md, consistent use of spaces instead of tabs, better examples

src/Concurrency.php

@@ -0,0 +1,47 @@
+<?php
+
+namespace Toalett\Multiprocessing;
+
+use Toalett\Multiprocessing\Exception\InvalidArgumentException;
+
+class Concurrency
+{
+    private const VALUE_UNLIMITED = -1;
+    private int $limit;
+
+    private function __construct(int $limit)
+    {
+        if ($limit === 0 || $limit < self::VALUE_UNLIMITED) {
+            throw new InvalidArgumentException('-1 or positive integer', $limit);
+        }
+        $this->limit = $limit;
+    }
+
+    public static function singleWorker(): self
+    {
+        return new self(1);
+    }
+
+    public static function atMost(int $limit): self
+    {
+        return new self($limit);
+    }
+
+    public static function unlimited(): self
+    {
+        return new self(self::VALUE_UNLIMITED);
+    }
+
+    public function isUnlimited(): bool
+    {
+        return $this->limit === self::VALUE_UNLIMITED;
+    }
+
+    public function isReachedBy(int $amount): bool
+    {
+        if ($this->isUnlimited()) {
+            return false;
+        }
+        return $amount >= $this->limit;
+    }
+}