adding info for the cost function

docs/guide/add-an-operator.md

@@ -26,30 +26,25 @@ This guide shows the *3 steps* that developers need to follow if they want to ad
 We use the Map operator as an example.
 
 ## Step 1: Add a Wayang operator
-Wayang operators are located under the ```wayang-basic``` in the ```org.apache.wayang.basic.operators``` package. 
+Wayang operators are located under the ```wayang-basic``` in the ```org.apache.wayang.basic.operators``` package. <br>
+An operator needs to extend from one of the following abstract classes: ```UnaryToUnaryOperator```, ```BinaryToUnaryOperator```, ```UnarySource```, ```UnarySink```.<br>
+For a unary to unary operator, see for example [here](https://github.com/apache/incubator-wayang/blob/main/wayang-commons/wayang-basic/src/main/java/org/apache/wayang/basic/operators/MapOperator.java). 
 
-An operator needs to extend from one of the following abstract classes: ```UnaryToUnaryOperator```, ```BinaryToUnaryOperator```, ```UnarySource```, ```UnarySink```.
-
-For a unary to unary operator, see for example here:
-https://github.com/apache/incubator-wayang/blob/main/wayang-commons/wayang-basic/src/main/java/org/apache/wayang/basic/operators/MapOperator.java
-
-For enhanced performance in Wayang, consider adding a cardinality estimator as well.
+For enhanced performance in Wayang, consider adding a cardinality estimator by overriding the ```createCardinalityEstimator()``` function as [here](https://github.com/apache/incubator-wayang/blob/main/wayang-commons/wayang-basic/src/main/java/org/apache/wayang/basic/operators/MapOperator.java#L112C43-L112C70).
 
 ## Step 2: Add the (platform-specific) execution operators
-Execution operators are located under the corresponding module of ```wayang-platforms```. For instance, Java execution operators are located in the ```org.apache.wayang.java.operators``` package of the ```wayang-java``` module.
-
-An execution operator needs to extend from its corresponding Wayang operator and implement the corresponding platform operator interface.
-
-For the above ```MapOperator```, the following is the corresponding ```JavaMapOperator```:
-https://github.com/apache/incubator-wayang/blob/main/wayang-platforms/wayang-java/src/main/java/org/apache/wayang/java/operators/JavaMapOperator.java
+Execution operators are located under the corresponding module of ```wayang-platforms```. For instance, Java execution operators are located in the ```org.apache.wayang.java.operators``` package of the ```wayang-java``` module.<br>
+An execution operator needs to extend from its corresponding Wayang operator and implement the corresponding platform operator interface.<br>
+For the above ```MapOperator```, the following is the corresponding [```JavaMapOperator```](https://github.com/apache/incubator-wayang/blob/main/wayang-platforms/wayang-java/src/main/java/org/apache/wayang/java/operators/JavaMapOperator.java).
 
-For enhanced performance in Wayang, consider adding a load function as well.
+For enhanced performance in Wayang, consider adding a load function as well:<br>
+For this you need to overwrite the ```getLoadProfileEstimatorConfigurationKey()``` function and provide the right key that will then be read from a properties file.
+For the JavaMapOperator it's: wayang.java.map.load. Then add in the corresponding properties file (e.g., [this](https://github.com/apache/incubator-wayang/blob/main/wayang-platforms/wayang-java/src/main/resources/wayang-java-defaults.properties) is for the java executor) the template which is the mathematical formula that represents the cost of this operator and an instantiation of it. See [here](https://github.com/apache/incubator-wayang/blob/main/wayang-platforms/wayang-java/src/main/resources/wayang-java-defaults.properties#L25) for the example of the map operator.
 
 ## Step 3: Add mappings
-Create mappings from the Wayang operator to the platform-specific execution operators. The mappings are located in the corresponding execution module in the ```org.apache.wayang.java.operators``` package.
+Create mappings from the Wayang operator to the platform-specific execution operators. <br>
+The mappings are located in the corresponding execution module in the ```org.apache.wayang.java.operators``` package.<br>
+For the above ```MapOperator``` and ```JavaMapOperator```, see [here](https://github.com/apache/incubator-wayang/blob/main/wayang-platforms/wayang-java/src/main/java/org/apache/wayang/java/mapping/MapMapping.java).
 
-For the above ```MapOperator``` and ```JavaMapOperator```, see here:
-https://github.com/apache/incubator-wayang/blob/main/wayang-platforms/wayang-java/src/main/java/org/apache/wayang/java/mapping/MapMapping.java
+After that you need to declare this mapping in Wayang in the corresponding [```Mappings```](https://github.com/apache/incubator-wayang/blob/main/wayang-platforms/wayang-java/src/main/java/org/apache/wayang/java/mapping/Mappings.java#L37) class.
 
-After that you need to declare this mapping in the Wayang in the corresponding ```Mappings``` class:
-https://github.com/apache/incubator-wayang/blob/main/wayang-platforms/wayang-java/src/main/java/org/apache/wayang/java/mapping/Mappings.java#L37