Add more description to mock CSI driver

mock/README.md

@@ -1,2 +1,22 @@
 # Mock CSI Driver
-Extremely simple mock driver used to test `csi-sanity` based on `rexray/gocsi/mock`
+Extremely simple mock driver used to test `csi-sanity` based on `rexray/gocsi/mock`.
+It can be used for testing of Container Orchestrators that implement client side
+of CSI interface.
+
+```
+Usage of mock:
+  -disable-attach
+        Disables RPC_PUBLISH_UNPUBLISH_VOLUME capability.
+  -name string
+        CSI driver name. (default "io.kubernetes.storage.mock")
+```
+
+It prints all received CSI messages to stdout encoded as json, so a test can check that
+CO sent the right CSI message.
+
+Example of such output:
+
+```
+gRPCCall: {"Method":"/csi.v0.Controller/ControllerGetCapabilities","Request":{},"Response":{"capabilities":[{"Type":{"Rpc":{"type":1}}},{"Type":{"Rpc":{"type":3}}},{"Type":{"Rpc":{"type":4}}},{"Type":{"Rpc":{"type":6}}},{"Type":{"Rpc":{"type":5}}},{"Type":{"Rpc":{"type":2}}}]},"Error":""}
+gRPCCall: {"Method":"/csi.v0.Controller/ControllerPublishVolume","Request":{"volume_id":"12","node_id":"some-fake-node-id","volume_capability":{"AccessType":{"Mount":{}},"access_mode":{"mode":1}}},"Response":null,"Error":"rpc error: code = NotFound desc = Not matching Node ID some-fake-node-id to Mock Node ID io.kubernetes.storage.mock"}
+```