Add initial 2.1 design doc

This document contains a skeleton for the 2.1 design process.
For now it just has introductory paragraphs and a structure for the
various areas' design, but some sections still don't have a text, as
we're still in the early design phases.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>

doc/design-2.1.rst

+=================
+Ganeti 2.1 design
+=================
+
+This document describes the major changes in Ganeti 2.1 compared to
+the 2.0 version.
+
+The 2.1 version will be a relatively small release. Its main aim is to avoid
+changing too much of the core code, while addressing issues and adding new
+features and improvements over 2.0, in a timely fashion.
+
+.. contents:: :depth: 3
+
+Objective
+=========
+
+Ganeti 2.1 will add features to help further automatization of cluster
+operations, further improbe scalability to even bigger clusters, and make it
+easier to debug the Ganeti core.
+
+Background
+==========
+
+Overview
+========
+
+Detailed design
+===============
+
+As for 2.0 we divide the 2.1 design into three areas:
+
+- core changes, which affect the master daemon/job queue/locking
+- logical unit/feature changes
+- external interface changes (eg. command line, os api, hooks, ...)
+
+Core changes
+------------
+
+Feature changes
+---------------
+
+External interface changes
+--------------------------
+