added README and fixed abstract method for Specification

Author: Dominik Liebler

Specification/AbstractSpecification.php

@@ -13,9 +13,7 @@ abstract class AbstractSpecification implements SpecificationInterface
      * 
      * @return bool
      */
-    public function isSatisfiedBy(Item $item)
-    {
-    }
+    abstract public function isSatisfiedBy(Item $item);
 
     /**
      * Creates a new logical AND specification

Specification/README.md

@@ -0,0 +1,7 @@
+# Specification
+
+## Purpose
+
+Builds a clear specification of business rules, where objects can be checked against. The composite specification class has
+one method called `isSatisfiedBy` that returns either true or false depending on whether the given object satisfies the specification.
+