Add single task execution docs

rsts/user/features/index.rst

@@ -14,3 +14,4 @@ Flyte Features
     launchplans
     task_cache
     roles
+    single_task_execution

rsts/user/features/single_task_execution.rst

@@ -0,0 +1,90 @@
+.. _features-singletaskexec:
+
+Single Task Excution
+####################
+
+What are single task executions?
+================================
+
+Tasks are the most atomic unit of execution in Flyte.  Although workflows are traditionally composed of multiple tasks with dependencies
+defined by shared inputs and outputs it can be helpful to execute a single task during the process of iterating on its definition.
+It can be tedious to write a new workflow definition every time you want to excecute a single task under development but single task
+executions can be used to easily iterate on task logic.
+
+Launch a single task
+====================
+
+After you've built an image with your updated task code, create an execution using launch:
+
+.. code-block:: python
+
+     @inputs(plant=Types.String)
+     @outputs(out=Types.String)
+     @python_task
+     def my_task(wf_params, plant, out)
+         ...
+
+
+     my_single_task_execution = my_task.launch(project="my_flyte_projext", domain="development", inputs={'plant': 'ficus'})
+     print("Created {}".format(my_single_task_execution.id))
+
+Just like workflow executions, you can optionally pass a user-defined name, labels, annotations, and/or notifications when launching a single task.
+
+The type of ``my_single_task_execution`` is `SdkWorkflowExecution <https://github.com/lyft/flytekit/blob/1926b1285591ae941d7fc9bd4c2e4391c5c1b21b/flytekit/common/workflow_execution.py#L14>`_
+and has the full set of methods and functionality available for conventional WorkflowExecutions.
+
+
+Fetch and launch a single task
+==============================
+
+Single task executions aren't limited to just tasks you've defined in your code. You can reference previously registered tasks and launch a single task execution like so:
+
+.. code-block:: python
+
+     from flytekit.common.tasks import task as _task
+
+     my_task = _task.SdkTask.fetch("my_flyte_project", "production", "workflows.my_task", "abc123")  # project, domain, name, version
+
+     my_task_exec = my_task.launch(project="my_other_project", domain="development", inputs={'plant': 'philodendron'})
+     my_task_exec.wait_for_completion()
+
+
+Launch a single task from the commandline
+=========================================
+
+Previously registered tasks can also be launched from the command-line using :ref:`flyte-cli <features-flytecli>`
+
+.. code-block:: console
+
+    $ flyte-cli -h example.com -p my_flyte_project -d development launch-task \
+        -u tsk:my_flyte_project:production:my_complicated_task:abc123 -- an_input=hi \
+        other_input=123 more_input=qwerty
+
+
+Monitoring single task executions in the Flyte console
+======================================================
+
+Single task executions don't yet have native support in the Flyte console but they're accessible using the same URLs as ordinary workflow executions.
+
+For example, for a console hosted example.com you can visit ``example.com/console/projects/<my_project>/domains/<my_domain>/executions/<execution_name>`` to track the progress of 
+your execution. Log links and status changes will be available as your execution progresses.
+
+
+Registering and launching a single task
+=======================================
+
+A certain category of tasks don't rely on custom containers with registered images to run. Therefore, you may find it convenient to use
+``register_and_launch`` on a task definition to immediately launch a single task execution, like so:
+
+.. code-block:: python
+
+    containerless_task = SdkPrestoTask(
+        task_inputs=inputs(ds=Types.String, count=Types.Integer, rg=Types.String),
+        statement="SELECT * FROM flyte.widgets WHERE ds = '{{ .Inputs.ds}}' LIMIT '{{ .Inputs.count}}'",
+        output_schema=Types.Schema([("a", Types.String), ("b", Types.Integer)]),
+        routing_group="{{ .Inputs.rg }}",
+    )
+
+    my_single_task_execution = containerless_task.register_and_launch(project="my_flyte_projext", domain="development",
+        inputs={'ds': '2020-02-29', 'count': 10, 'rg': 'my_routing_group'})
+