Add explicit README for configstore future goals.

Previously, supposedly, each individual client of configstore
was supposed to add the add_hwservice attribute itself to get
ahold of the specific sub-interface of configstore relevant to
it. However, there is only one configstore interface,
ISurfaceFlingerConfigs.

From this point onward, the configstore hal is to be thought of
as specifically relating to surface flinger. Other properties
may be added as other attributes/packages.

For instance, if we want a configstore entry for 'IFooConfig',
then we would add the configuration to one of the following
packages:
- android.hardware.foo@X.Y (to the interface itself)
- android.hardware.foo.config@X.Y (to a configuration of the interface)
- android.hardware.configstore.foo@X.Y (as a sub-interface of configstore)

and then it could be associated with the sepolicy attributes
(respectively):
- hal_foo
- hal_foo_config (or just hal_foo if they are 1-1)
- hal_configstore_foo

The specific pattern to be followed irrelevant to this CL
and subject to future discussion, the point being that we're going
to have a separate sepolicy attribute (and package, although this
isn't strictly necessary) for each separate domain's configuration.

Bug: 109806245
Test: N/A
Change-Id: I837e0bdd11e028265756a905c234e087924c1d6b