add blurb to declare the migration of CB core to sygma core

[haochizzle]

Description

per request from @itsbobbyzzz168 and @alexmmueller, adding a tiny blurb to notify devs of migration of chainbridge-core to sygma-core, which will be done by @mpetrun5 soon.

Related Issue Or Context

@itsbobbyzzz168 noticed "chainbridge" repo is referenced in snowbridge's proposal. @itsbobbyzzz168 also mentions the chainbridge-core repo is mentioned in his newest treasury proposal. we want to mention sygma to show that work is ongoing and that the repo is still in active development.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation

Checklist:

• I have commented my code, particularly in hard-to-understand areas.
• I have ensured that all acceptance criteria (or expected behavior) from issue are met
• I have updated the documentation locally and in chainbridge-docs.
• I have added tests to cover my changes.
• I have ensured that all the checks are passing and green, I've signed the CLA bot

[github-actions]

Go Test coverage is 65.2 %\ ✨ ✨ ✨

[mpetrun5]

Will create a proper archive deprecation when we stop using it in the MPC relayer.