Update CONTRIBUTING.md for benchmarking instructions.

CONTRIBUTING.md

@@ -8,3 +8,18 @@ license](https://swift.org/LICENSE.txt).
 Before submitting the pull request, please make sure you have tested your
 changes and that they follow the Swift project [guidelines for contributing
 code](https://swift.org/contributing/#contributing-code).
+
+---
+
+For benchmarking of particular algorithms, please use the 
+
+`algorithms-benchmark` executable target. For convenience, this target 
+
+already imports `Algorithms`.
+
+Example usage:
+
+`$ swift run -c release algorithms-benchmark run <name-of-results-file> --cycles 5`
+
+`$ swift run -c release algorithms-benchmark render <name-of-results-file> <name-of-chart-file>.png`
+