Update downsample.md

doc/downsample.md

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 # Downsampling the Training Data
 
+Downsampling is a very simple way to improve the efficiency of your evolutionary runs. It might allow for deeper evolutionary searches and a greater success rate.
+
+Using Downsampled-Lexicase selection with propeller is easy:
+
+Set the :parent-selection argument to :ds-lexicase as follows
 ```clojure
-:downsample-rate
+lein run -m propeller.problems.simple-regression :parent-selection :ds-lexicase <ARGS>
 ```
+
+Arguments:
+
+
+- Case Downsampling function:
+    - Random sampling (default)
+    - Case tournament selection
+         ```clojure 
+        :ds-function :case-tournament 
+        ```
+    - Case Lexicase Selection
+        WIP
+- Downsample Rate:
+    ```clojure 
+        :downsample-rate 0.1
+    ```    
+