[doc] document ECS2DCS2ECS mock server usage

OCTRL-1026

Author: knopers8

README.md

@@ -110,6 +110,7 @@ There are two ways of interacting with AliECS:
         * [DCS PrepareForRun behaviour](/core/integration/README.md#dcs-prepareforrun-behaviour)
         * [DCS StartOfRun behaviour](/core/integration/README.md#dcs-startofrun-behaviour)
         * [DCS EndOfRun behaviour](/core/integration/README.md#dcs-endofrun-behaviour)
+        * [ECS2DCS2ECS mock server](/core/integration/README.md#ecs2dcs2ecs-mock-server)
       * [DD Scheduler](/core/integration/README.md#dd-scheduler)
       * [Kafka (legacy)](/core/integration/README.md#kafka-legacy)
       * [ODC](/core/integration/README.md#odc)

core/integration/README.md

@@ -138,6 +138,19 @@ The environment will then declare a **failure**, the
 `STOP_ACTIVITY` transition will be blocked and the environment
 will move to `ERROR`.
 
+### ECS2DCS2ECS mock server
+
+ECS2DCS2ECS is a simple server that mimics the DCS service and is developed by the DCS team.
+In particular, it sends heartbeats, reports detectors as PFR or SOR available and replies with requested state sequences.
+
+The state sequences should be added as extra arguments to the DCS calls with key `"STATE_SEQUENCE"`, while the value is a comma-separated list of `MILLIS_DELAY:DETECTOR_STATE` values.
+
+From the AliECS user point of view, state sequences can be defined as the following user-vars (Advanced Configuration panel in COG), which are then passed to the corresponding PFR/SOR/EOR calls for all detectors:
+
+- `dcs_pfr_state_sequence`
+- `dcs_sor_state_sequence`
+- `dcs_eor_state_sequence`
+
 ## DD Scheduler
 
 DD scheduler plugin informs the Data Distribution software about the pool of FLPs taking part in data taking.