document parallel execution

doc/index.rst

@@ -35,5 +35,6 @@ Documentation
 .. toctree::
     :maxdepth: 2
 
+    parallel
     actors
     api

doc/parallel.rst

+# Parallel execution
+
+Options for parallel execution can be provided when executing a workflow
+
+.. code:: python
+
+    from ewoksppf import execute_graph
+
+    result = execute_graph("/path/to/graph.json", pool_type=..., scaling_workers=..., max_workers=...)
+
+The options are
+
+  * `pool_type`: _thread_, _process_ (default), _ndprocess_, _multiprocessing_,
+                 _ndmultiprocessing_, _billiard_, _gevent_ (default)
+  * `scaling_workers`: _True_ (default) or _False_
+  * `max_workers`: only applies when _scaling_workers=False_
+  * `context`: _spawn_ or _fork_ (only applies when using processes)