cg: Add documentation about props

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

Documentation/criu.txt

@@ -151,6 +151,46 @@ In other words, do not use it until really needed.
     Without this argument *criu* will not save cgroups configuration
     associated with a task.
 
+*--cgroup-props* '<spec>'::
+    Specify controllers and their properties to be carried into the
+    image file. *criu* predefines specifications for common controllers
+    but since the kernel can add new controllers and modify their
+    properties there should be a way to specify ones matched the kernel.
++
+'<spec>' describes the controller and properties specification in
+simplified YAML form:
++
+----------
+"c1":
+ - "strategy": "merge"
+ - "properties": ["a", "b"]
+"c2":
+ - "strategy": "replace"
+ - "properties": ["c", "d"]
+----------
++
+Where 'c1' and 'c2' are controllers names, and 'a', 'b', 'c', 'd' are
+their properties.
++
+Mark the format: double quotes, spaces and new lines are required.
+The '<strategy>' specify how to behave if controller specified already
+exsit as built in one: *criu* either 'merge' or 'replace' them.
++
+Thus for command line argument the example above will look like:
++
+----------
+--cgroup-props "\"c1\":\n - \"strategy\": \"merge\"\n - \"properties\": [\"a\", \"b\"]\n \"c2\":\n - \"strategy\": \"replace\"\n - \"properties\": [\"c\", \"d\"]"
+----------
+
+*--cgroup-props-file* '<path>'::
+    Same as *--cgroup-props* except the specification is read from
+    a file pointed by '<path>'.
+
+*--cgroup-props-ignore-default*::
+    When combined with *--cgroup-props* makes *criu* to substitute
+    predefined controller property with new one shipped. Otherwise
+    predefined properies are merged with provided.
+
 *--tcp-established*::
     Checkpoint established TCP connections.