added howto "Adding a progress bar to nornir" (#381)

* added howto about how to add a progress bar to nornir

* undo change

docs/howto/progress_bar.rst

@@ -0,0 +1,49 @@
+Adding a progress bar
+=====================
+
+In this how to we want to show an example on how to integrate ``nornir`` with `tqdm <https://tqdm.github.io/>`_ to add a nice way of tracking progress of our script without having to print the results on screen. First, this is how the result will look like when we run the script:
+
+.. image:: progress_bar/demo.gif
+
+And now the code::
+
+   from nornir import InitNornir
+   from nornir.plugins.tasks.networking import napalm_get
+
+   from tqdm import tqdm
+
+   nr = InitNornir(config_file="config.yaml")
+
+
+   def multiple_progress_bar(task, napalm_get_bar, other_bar):
+       """
+       This task takes two paramters that are in fact bars;
+       napalm_get_bar and other_bar. When we want to tell
+       to each respective bar that we are done and should update
+       the progress we can do so with bar_name.update()
+       """
+       task.run(task=napalm_get, getters=["facts"])
+       napalm_get_bar.update()
+       tqdm.write(f"{task.host}: facts gathered")
+
+       # more actions go here
+       other_bar.update()
+       tqdm.write(f"{task.host}: done!")
+
+
+   # we create the first bar named napalm_get_bar
+   with tqdm(
+       total=len(nr.inventory.hosts), desc="gathering facts",
+   ) as napalm_get_bar:
+       # we create the second bar named other_bar
+       with tqdm(
+           total=len(nr.inventory.hosts), desc="other action   ",
+       ) as other_bar:
+           # we call our grouped task passing both bars
+           nr.run(
+               task=multiple_progress_bar,
+               napalm_get_bar=napalm_get_bar,
+               other_bar=other_bar,
+           )
+
+It looks a bit daunting due to the nesting but basically we are creating two progress bars, one inside the other and then we just pass them to our grouped task. Finally, the grouped task can update the progress by calling ``bar_name.update()``. You can also add further information using ``tqdm.write("more info!")`` if you want.