Add an 'index' of design documents

This will be an overview document, enumerating the changes without going
into details and pointing to the actual documents.

Reviewed-by: ultrotter

doc/design-2.0-index.rst

+Ganeti 2.0 design documents
+===========================
+
+
+The 2.x versions of Ganeti will constitute a rewrite of the 'core'
+architecture, plus some additional features (however 2.0 is geared
+toward the core changes).
+
+Core changes
+------------
+
+The main changes will be switching from a per-process model to a
+daemon based model, where the individual gnt-* commands will be
+clients that talk to this daemon (see the design-2.0-master-daemon
+document). This will allow us to get rid of the global cluster lock
+for most operations, having instead a per-object lock (see
+design-2.0-granular-locking). Also, the daemon will be able to queue
+jobs, and this will allow the invidual clients to submit jobs without
+waiting for them to finish, and also see the result of old requests
+(see design-2.0-job-queue).
+
+Beside these major changes, another 'core' change but that will not be
+as visible to the users will be changing the model of object attribute
+storage, and separate that into namespaces (such that an Xen PVM
+instance will not have the Xen HVM parameters). This will allow future
+flexibility in defining additional parameters.
+
+
+Functionality changes
+---------------------
+
+The disk storage will receive some changes, and will also remove
+support for the drbd7 and md disk types. See the
+design-2.0-disk-changes document.
+
+The configuration storage will be changed, with the effect that more
+data will be available on the nodes for access from outside ganeti
+(e.g. from shell scripts) and that nodes will get slightly more
+awareness of the cluster configuration.
+
+The RAPI will enable modify operations (beside the read-only queries
+that are available today), so in effect almost all the operations
+available today via the ``gnt-*`` commands will be available via the
+remote API.
+
+A change in the hypervisor support area will be that we will support
+multiple hypervisors in parallel in the same cluster, so one could run
+Xen HVM side-by-side with Xen PVM on the same cluster.
+
+New features
+------------
+
+There will be a number of minor feature enhancements targeted to
+either 2.0 or subsequent 2.x releases:
+
+- multiple disks, with custom properties (read-only/read-write, exportable,
+  etc.)
+- multiple NICs