vtkThreadedCallbackQueue: adding a .md file

Documentation/release/dev/add-ThreadedCallbackQueue.md

+## Add vtkThreadedCallbackQueue
+
+`vtkThreadedCallbackQueue` is a new utility in VTK that allows to run functions in the background on
+different threads. They are popped from the queue and invoked when a thread is available.
+You can add any kind of functions with parameters to the queue using the `Push` method.
+Controls are provided to change the number of threads, start and stop the queue. Note that all
+controls run asynchronously. For example, the `Stop` method will likely end before the queue
+is actually
+stopped. This allows to change this queue's state without blocking the main pipeline. If it were not
+the case, a heavy job currently running could make calling `Stop` potentially very long to execute.
+Controls are serially run in the background by a dedicated thread.