Adding StopTask endpoint and endpoint documentation

libdaggy/src/DAGRunner.cpp

@@ -90,6 +90,11 @@ namespace daggy {
     }
   }
 
+  void DAGRunner::stopTask(const std::string &taskName)
+  {
+    executor_.stop(runID_, taskName);
+  }
+
   void DAGRunner::queuePending()
   {
     if (!running_)