chore: update readme for container resource detector

Update README of Container Resource Detector to clarify the source of the data.

Part Of #2055

Signed-off-by: maryliag <[email protected]>

Author: maryliag

detectors/node/opentelemetry-resource-detector-container/README.md

@@ -28,7 +28,13 @@ const tracerProvider = new NodeTracerProvider({ resource });
 
 ## Available detectors
 
-- `containerDetector`: Populates `container.id` for processes running on containers supporting : docker( cgroup v1 or v2 ) or with containerd
+### ContainerDetector
+
+Populates `container.id` for processes running on containers supporting : docker( cgroup v1 or v2 ) or with containerd
+
+| Resource Attribute  |  Description                                                    |
+|---------------------|-----------------------------------------------------------------|
+| container.id        | Value parsed from file `/proc/self/cgroup` (cgroup v1). If it doesn't exist, parse the value from file `/proc/self/mountinfo` (cgroup v2)|
 
 ## Useful links