add readme to the remotecluster and sharedchannel service directories (#30123)

server/platform/services/sharedchannel/README.md

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+## Shared Channel Service
+
+Package `sharedchannel` implements Mattermost's shared channels functionality, for sharing channel content across Mattermost instances/clusters. Here are the key responsibilities:
+
+### Channel Sharing:
+
+- Allows channels to be shared between different Mattermost instances/clusters
+- Handles inviting remote clusters to shared channels
+- Manages permissions and read-only status for shared channels
+
+### Content Synchronization:
+
+- Syncs posts, reactions, user profiles, and file attachments between instances
+- Handles permalink processing between instances
+- Manages user profile images sync
+- Maintains sync state and cursors to track what has been synchronized
+
+### Remote Communication:
+
+- Processes incoming sync messages from remote clusters
+- Sends updates to remote clusters when local changes occur
+- Handles connection state changes with remote clusters
+- Manages retry logic for failed sync attempts
+
+### Security:
+
+- Validates permissions for shared channel operations
+- Ensures users can only sync content they have access to
+- Verifies remote cluster authenticity
+- Sanitizes user data during sync
+
+The service acts as a bridge between Mattermost instances, allowing users from different instances to collaborate in shared channels while keeping content synchronized across all participating instances.
+
+This is implemented through a Service struct that handles all the shared channel operations and maintains the synchronization state. It works in conjunction with the RemoteCluster service to handle the actual communication between instances.