Merge branch 'simplestreams0' into 'master'

add information on image metadata/simplestream

See merge request !1

web/apps/juju.rst

@@ -248,3 +248,4 @@ Additional notes on `Juju`
 --------------------------
 - `Juju` by default stores its configuration and state locally in the ``~/.local/share/juju`` directory. This includes the ``ssh`` subdirectory, which contains the admin SSH keys. This can be useful for e.g. debugging purposes.
 
+- When `Juju` deploys its applications, the `charms` are fetched remotely (from e.g. https://jujucharms.com) but the images are not: `Juju` queries the local keystone about the ``product-streams`` endpoint, which supplies metadata to retrieve the images from the local `Glance` service. One implementation of the ``product-streams`` endpoint is supplied by `SimpleStreams`.