docs: update filename of README.md in .github

[bang9]

External Contributions

This project is not yet set up to accept pull requests from external contributors.

If you have a pull request that you believe should be accepted, please contact
the Developer Relations team developer-advocates@sendbird.com with details
and we'll evaluate if we can setup a CLA to allow for the contribution.

For Internal Contributors

레포 문서로 사용되는 README.md 의 우선순위가 다음과 같아서 수정을 합니다.

.github/README.md -> root README.md -> docs/READEME.md

Description Of Changes

Add a brief description of the changes that you have involved in this PR

Types Of Changes

What types of changes does your code introduce to this project?
Put an x in the boxes that apply_

• Bugfix
• New feature
• Documentation (correction or otherwise)
• Cosmetics (whitespace, appearance (ex) Prettier)
• Build configuration
• Improvement (refactor code)
• Test

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 10.89%. Comparing base (b2fb8c3) to head (7552a12).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #271   +/-   ##
=======================================
  Coverage   10.89%   10.89%           
=======================================
  Files         360      360           
  Lines        9016     9016           
  Branches     2419     2419           
=======================================
  Hits          982      982           
  Misses       8033     8033           
  Partials        1        1           

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