Update Documentation for Deprecated Features

[screwtape151]

Issue

The README currently contains warnings regarding the deprecation of Atlas Device Sync and Realm SDKs. However, there is no information on the alternatives or how existing users should migrate away from these features.

Suggested Improvements

1. Migration Guide: Include a section in the README or a link to a dedicated migration guide for users transitioning from the deprecated features to the community version of RealmJS.
2. Community Support: Provide resources or links to the community where users can ask questions regarding the migration process.
3. Deprecation Timeline: A clearer timeline or FAQs concerning the deprecation could help users understand what steps they need to take and by when.

These improvements would greatly aid users who are currently utilizing the deprecated features and need guidance moving forward.