Adding StopTask endpoint and endpoint documentation

libdaggy/include/daggy/DAGRunner.hpp

@@ -31,6 +31,7 @@ namespace daggy {
     TaskDAG run();
     void resetRunning();
     void stop(bool kill = false, bool blocking = false);
+    void stopTask(const std::string &taskName);
 
   private:
     void collectFinished();