App registry (#1979)

* Add app level registry

* Add documentation for app registry

* Remove unused import

* Add force_create keyword to Sanic.get_app

* Add force_commit to docs

docs/sanic/getting_started.rst

@@ -60,3 +60,26 @@ Open the address `http://0.0.0.0:8000 <http://0.0.0.0:8000>`_ in your web browse
 the message *Hello world!*.
 
 You now have a working Sanic server!
+
+5. Application registry
+-----------------------
+
+When you instantiate a Sanic instance, that can be retrieved at a later time from the Sanic app registry. This can be useful, for example, if you need to access your Sanic instance from a location where it is not otherwise accessible.
+
+.. code-block:: python
+
+    # ./path/to/server.py
+    from sanic import Sanic
+
+    app = Sanic("my_awesome_server")
+
+    # ./path/to/somewhere_else.py
+    from sanic import Sanic
+
+    app = Sanic.get_app("my_awesome_server")
+
+If you call ``Sanic.get_app("non-existing")`` on an app that does not exist, it will raise ``SanicException`` by default. You can, instead, force the method to return a new instance of ``Sanic`` with that name:
+
+.. code-block:: python
+
+    app = Sanic.get_app("my_awesome_server", force_create=True)