[django] Adds ability to create a get_graph() command method as a generator for multiple, synchronous jobs execution in django command.

docs/tutorial/2-jobs.rst

@@ -4,7 +4,54 @@ Part 2: Writing ETL Jobs
 What's an ETL job ?
 :::::::::::::::::::
 
-In |bonobo|, an ETL job is a formal definition of an executable graph.
+In |bonobo|, an ETL job is a single graph that can be executed on its own.
+
+Within a graph, each node are isolated and can only communicate using their
+input and output queues. For each input row, a given node will be called with
+the row passed as arguments. Each *return* or *yield* value will be put on the
+node's output queue, and the nodes connected in the graph will then be able to
+process it.
+
+|bonobo| is a line-by-line data stream processing solution.
+
+Handling the data-flow this way brings the following properties:
+
+- **First in, first out**: unless stated otherwise, each node will receeive the
+  rows from FIFO queues, and so, the order of rows will be preserved. That is
+  true for each single node, but please note that if you define "graph bubbles"
+  (where a graph diverge in different branches then converge again), the
+  convergence node will receive rows FIFO from each input queue, meaning that
+  the order existing at the divergence point wont stay true at the convergence
+  point.
+
+- **Parallelism**: each node run in parallel (by default, using independant
+  threads). This is useful as you don't have to worry about blocking calls.
+  If a thread waits for, let's say, a database, or a network service, the other
+  nodes will continue handling data, as long as they have input rows available.
+
+- **Independance**: the rows are independant from each other, making this way
+  of working with data flows good for line-by-line data processing, but
+  also not ideal for "grouped" computations (where an output depends on more
+  than one line of input data). You can overcome this with rolling windows if
+  the input required are adjacent rows, but if you need to work on the whole
+  dataset at once, you should consider other software.
+
+Graphs are defined using :class:`bonobo.Graph` instances, as seen in the
+previous tutorial step.
+
+What can be a node?
+:::::::::::::::::::
+
+**TL;DR**: … anything, as long as it’s callable().
+
+Functions
+---------
+
+.. code-block:: python
+
+    def get_item(id):
+        return id, items.get(id)
+
 
 Each node of a graph will be executed in isolation from the other nodes, and the data is passed from one node to the
 next using FIFO queues, managed by the framework. It's transparent to the end-user, though, and you'll only use