chore: add readme for async destinaiton module

[ItsSudip]

Description

We are adding a readme to asyncdestinaiton manager to understand and onboard a new async integration.

Linear Ticket

https://linear.app/rudderstack/issue/INT-4136/create-read-me-for-async-destinations

Security

• The code changed/added as part of this pull request won't create any security issues with how the software is being used.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.70%. Comparing base (51dd4f7) to head (3c8c156).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6356      +/-   ##
==========================================
+ Coverage   77.63%   77.70%   +0.06%     
==========================================
  Files         523      523              
  Lines       70204    70204              
==========================================
+ Hits        54502    54550      +48     
+ Misses      12875    12825      -50     
- Partials     2827     2829       +2     

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