Add archive notice to README

[masih]

Update readme with notice of archive/move to sei-chain.

Relates to:

• Migrate sei-tendermint into sei-chain as mono repo sei-chain#2421

[github-actions]

The latest Buf updates on your PR. Results from workflow Buf / buf (pull_request).

Build	Format	Lint	Breaking	Updated (UTC)
✅ passed	✅ passed	✅ passed	✅ passed	Oct 2, 2025, 10:21 AM

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 57.12%. Comparing base (020476e) to head (7d87838).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #330      +/-   ##
==========================================
+ Coverage   57.00%   57.12%   +0.11%     
==========================================
  Files         254      254              
  Lines       33815    33815              
==========================================
+ Hits        19276    19316      +40     
+ Misses      13000    12962      -38     
+ Partials     1539     1537       -2     

see 12 files with indirect coverage changes

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