(WIP) Block storage docs

docs/topics/block_storage.rst

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+Working with block storage
+==========================
+To add persistent storage to your cloud environments, you would use block
+storage devices, namley volumes and volume snapshots. A volume is attached to
+an instance and mounted as a file system for use by the applications. A volume
+snapshot is a point-in-time snapshot of a volume that can be shared with other
+cloud users. Before a snapshot can be used, it is necessary to create a volume
+from it.
+
+Volume storage
+--------------
+Operations, such as creating a new volume and listing the eixsting ones, are
+perfomed via the :class:`.VolumeService`.
+
+.. code-block:: python
+    vol = provider.block_store.volumes.create('Cloudbridge-vol', 1, 'us-east-1a')
+    vol.wait_till_ready()
+    provider.block_store.volumes.list()