Added bootstrap.rst

doc/source/structure.rst

 Structure of the infra.git repository
 =====================================
 
 Please also read the root README.md file in the source tree. The following
 sections contain an overview of the directory layout. All paths are relative to
 the root one, which means that infra/ refers to infra/infra/.
 
 Entry points
 ------------
 * run.py: wrapper script to run programs contained in subdirectories without
   having to deal with sys.path modifications. See README.md for details and
   invocation.
 * test.py: multi-purpose script to run tests.
 * infra/libs/: :doc:`reference/infra.libs` generally useful functions and classes
 * infra/services/: standalone programs intended to be run as daemons (more
   details below)
 * infra/tools: command-line tools, intended to be run by developers.
 * appengine/: all Chrome-infra-managed appengine applications (more details
   below)
 * infra/experimental: for, well, experimental stuff. Once they are stabilized,
   they should be moved in a more permanent place.
 
 
 Miscellaneous technical stuff
 -----------------------------
-* bootstrap/: utilities to set up a proper Python virtual environment.
+* bootstrap/: utilities to set up a proper Python virtual environment. More
+  details about it can be found on this page: :doc:`bootstrap`.
 * infra/path_hacks: submodules of this modules give access to modules in the
   build/ repository (``from infra.path_hacks.common import <stg>`` is actually
   getting ``<stg>`` from build/scripts/common).
 * utils/: purpose?
 * misc/: purpose? difference with utils/?
 
 infra/services/
 ---------------
 * gnumbd: git numbering daemon. Adds a monotonically-increasing number to git
   commits.
 * gsubtreed: git subtree daemon. Mirrors subdirectories of some repositories
   into independent repos so they can be consumed by downstream projects.
 * lkgr_finder / lkgr_tag_pusher: computes last known good revision, based on
   test results.
 
 appengine/ and appengine_modules/
 ---------------------------------
-Contains all appengine applications.
+appengine/ should only contain appengine applications (i.e. directories
+containing an app.yaml file), appengine_modules/ should contain python packages
+that can be shared between appengine apps. The correct way to use one of them is
+to symlink the directory inside the application directory. 
 
+Example: the `myapp` application should live in `appengine/myapp`. To use
+`appengine_module/testing_utils`, create a symlink to it in
+`appengine/myapp/testing_utils`. The name should remain the same as Python
+relies on directory names for its import system.
+