docs(propagation): add README.md for schema Field documentation propagation action (#14180)

datahub-actions/src/datahub_actions/plugin/action/propagation/docs/README.md

@@ -0,0 +1,84 @@
+# Documentation Propagation Action
+
+The Documentation Propagation Action allows you to automatically propagate documentation from schema fields to related schema fields. For example, when you add or update documentation for a column in a dataset, this action can automatically propagate that documentation to upstream, downstream, or sibling columns.
+
+## Functionality
+
+This action listens for documentation changes on schema fields and propagates those changes to related fields based on your configuration. It supports:
+
+- **Downstream Propagation**: Propagate documentation to columns in downstream datasets
+- **Upstream Propagation**: Propagate documentation to columns in upstream datasets
+- **Sibling Propagation**: Propagate documentation to columns in sibling datasets (e.g., views of the same table)
+
+## Configuration Options
+
+The Documentation Propagation Action provides several configuration options:
+
+- `enabled`: Controls whether documentation propagation is enabled (default: `true`)
+- `columns_enabled`: Controls whether column-level documentation propagation is enabled (default: `true`)
+- `datasets_enabled`: Controls whether dataset-level documentation propagation is enabled (default: `false`) - Note: Currently not implemented
+- `column_propagation_relationships`: Specifies which relationships to use for propagation. Valid values are:
+  - `UPSTREAM`: Propagate to upstream columns
+  - `DOWNSTREAM`: Propagate to downstream columns
+  - `SIBLING`: Propagate to sibling columns
+- `max_propagation_depth`: Maximum depth for propagation chains (default: `5`)
+- `max_propagation_fanout`: Maximum number of entities to propagate to in a single hop (default: `1000`)
+- `max_propagation_time_millis`: Maximum time in milliseconds for a propagation chain (default: `3600000` - 1 hour)
+
+## Example Configuration
+
+```yaml
+name: "documentation_propagation"
+source:
+  type: "kafka"
+  config:
+    connection:
+      bootstrap: ${KAFKA_BOOTSTRAP_SERVER:-localhost:9092}
+      schema_registry_url: ${SCHEMA_REGISTRY_URL:-http://localhost:8081}
+    # Topic Routing - which topics to read from.
+    topic_routes:
+      #mcl: ${METADATA_CHANGE_LOG_VERSIONED_TOPIC_NAME:-MetadataChangeLog_Versioned_v1} # Topic name for MetadataChangeLogEvent_v1 events.
+      pe: ${PLATFORM_EVENT_TOPIC_NAME:PlatformEvent_v1} # Topic name for PlatformEvent_v1 events.
+filter:
+  event_type: "EntityChangeEvent_v1"
+  event:
+    entityType: "schemaField"
+    category: "DOCUMENTATION"
+action:
+  type: "doc_propagation"
+  config:
+    enabled: true
+    columns_enabled: true
+    max_propagation_depth: 3 # Optional: Limit propagation depth
+
+datahub:
+  server: ${DATAHUB_GMS_HOST:-http://localhost:8080}
+  token: ${DATAHUB_GMS_TOKEN}
+```
+
+## Behavior
+
+When a documentation change is detected on a schema field:
+
+1. The action checks if propagation is enabled and if the change should be propagated
+2. It determines the appropriate propagation relationships based on your configuration
+3. It finds related schema fields based on those relationships
+4. It propagates the documentation to those fields, preserving attribution information
+5. It respects propagation limits (depth, fanout, time) to prevent excessive propagation
+
+## Propagation Attribution
+
+When documentation is propagated, the action adds attribution metadata to track:
+
+- The original source of the documentation
+- The propagation path
+- The time of propagation
+- The depth of propagation
+
+This attribution information is stored with the propagated documentation and can be viewed in the DataHub UI.
+
+## Caveats and Limitations
+
+- **Upstream Propagation**: When propagating upstream, the action only propagates if there is exactly one upstream field. This prevents ambiguous propagation when multiple upstream fields exist.
+- **Dataset Documentation**: Dataset-level documentation propagation is not currently supported, only schema field (column) documentation.
+- **Single Upstream Field**: For downstream propagation, the action only propagates to a downstream field if it has exactly one upstream field (the field being propagated from). This ensures that documentation is only propagated in clear one-to-one relationships.