DOC: added appengine.rst

doc/source/appengine.rst

+Developing an appengine application
+===================================
+
+Structure in infra.git
+----------------------
+
+Infra.git hosts several google AppEngine projects, located in the ``appengine/``
+directory. Each subdirectory of ``appengine/`` is supposed to be a single
+AppEngine application. Code shared between applications should live in
+``appengine_modules/`` and be symlinked into application directories (the
+AppEngine upload script ``appcfg.py`` follows symbolic links).
+
+
+Testing of AppEngine applications
+---------------------------------
+Tests included in AppEngine applications (classes deriving from
+``unittest.TestCase``) are run by ``test.py``. Some convenience functions to
+help using the testbed server are included in
+``appengine_modules/testing_utils``. Some examples can be found in existing
+applications.
+
+
+Managing AppEngine apps
+-----------------------
+A convenience script wrapping ``appcfg.py`` called ``gae.py`` can be used to
+simplify and normalize the deployment process in ``infra.git``. Just add a
+symlink to it in your application. It is located in
+``appengine/swarming/appengine/components/tools/gae.py``, but this location is
+bound to change soon(c) (2014-12-05).
+