Highlight that one can do e.g. `conn.compute.get('/servers')`. You
almost certainly *don't want* to do this, but it can be helpful (see:
our support for Nova's os-hypervisors API in OSC).

The flow of the README is modified slightly so we go sequentially from
high-level layers to low-level layers. Rubrics (header-like elements
that don't produce anchors or appear in tables of contents) are also
added to produce improve information hierarchy.

Change-Id: Ifd4a5a2c753f6698fa4384a197e81cc5383ef312
Signed-off-by: Stephen Finucane <stephenfin@redhat.com>